The following advice is for vanilla 1.09 Grand Campaign Byzantium, especially at very hard difficulty. Byzantium is assumed to be a two-province nation. What is great about an early game of Byzantium is that things never work out the same way in any two games. The following is only a guide. It is up to the player to improvise quite a bit.
Do not Fight the Turks in 1419
In the 1419 scenario, Byzantium is a small country with a wealth of core provinces. From the start, there are many worthy war targets to choose from. Initially, however, Ottoman Empire is not a good target for war. Certainly, many players of Byzantium want the Ottoman Empire out of their lands. However, choosing to fight an early war against Ottoman Empire is misguided. Ottoman Empire outnumbers Byzantium's troops by a three-to-one ratio. Any attempt to block the straits and keep the main component of Ottoman Empire's troops in Asia Minor is not likely to succeed for long because Ottoman Empire has superiority at sea as well as at land. Ottoman Empire only builds more ships and troops to replenish any losses. Much of Ottoman Empire's fresh troops are cavalry. Also, Ottoman Empire is aggressive in peace deals. Ottoman Empire will not likely accept any peace offer a player proposes unless the player is sacrificing 51% in war score. Peace is generally only acceptable on Ottoman Empire's terms. Any partial victory against Ottoman Empire is very costly; and the payoff is not likely that great compared to what can be achieved with much more glory.
A strong EU2 player will prefer to fight wars that result in profits not losses. If Byzantium takes loans to ensure victory against Ottoman Empire, the loans can only hamper the future success of Byzantium. Byzantium does not even have enough income to support her troops; interest payments likely lead to forced-loans, which may lead to bankruptcy. The beginning of the 1419 scenario is not the time for Byzantium to pick a fight with the biggest AI bully in the game. Profits from war with Ottoman Empire may later be had when the despised enemy has been damaged in other wars. Massive over-running allied victories are easily arranged in time for king and military leader Konstantinos XI to claim all for Byzantium. Until that time, Byzantium must build up a war chest. Some diplomatic strings must be pulled. Byzantium must also remain at least a two-province country.
There is perhaps a five-in-six possibility that Ottoman Empire will declare war on Byzantium in the first days of the game. Regardless of whether or not there is an early war against Ottoman Empire, the advice is still the same. Do not fight the Turks in 1419. In an early war against Ottoman Empire, Morea can be considered lost. To remain a two-province nation, Byzantium must look elsewhere for a province to own. Look also where there is money for the war chest.
Bosnia and Ragusa
Proposed targets for early war or wars are Bosnia and Ragusa. Bosnia and Ragusa often form an early alliance. Many times war with both Bosnia and Ragusa can be had when declaring war on Bosnia. Note that Ragusa will honor an alliance call, whereas Bosnia will not.
Bosnia makes a good addition to the Byzantine Empire. Although Bosnia is somewhat poor and has the wrong religion, Bosnia does have correct culture and is a core province of Byzantium. Because Bosnia is a core province, build capacity is higher and any country that temporarily sacks Constantinople is not able to demand peace on their terms. Also, war exhaustion is lowest in core provinces. Due to Byzantium's very low manpower, build capacity and war exhaustion may be factors in building just a few thousand more cavalry to recapture Byzantium's capital, should worst come to worst. Bosnia has an added advantage that it is a remote location from the Turks reach. If proper steps are taken to ensure the safety of the province, the Ottomans will not get military access to Bosnia, thereby preventing the Turks from obtaining a 100% victory.
For Byzantium, the most urgent war to get into is against Ragusa. Ragusa begins the scenario with 500d. Ragusa generally builds a warship near the start of the game. That means, for a number of months, Ragusa is unable to spend any more money recruiting troops for war. Ragusa has only 6K men to guard her country. Ragusa is an easy place to pick up around 400d in peace reparations.
Alliance with Serbia
When fighting Bosnia and Ragusa, an alliance with Serbia is most useful. In an allied war with Serbia against Bosnia and or Ragusa, Serbia provides not only troops but also land access and supply to Byzantium's armies in neighboring provinces of the enemy. Also, Serbia is very interested in annexing Bosnia, thereby absorbing the hits to badboy and hits to relations with Catholic nations. In the case that Serbia is allowed to annex Bosnia, another war against Serbia is required for Byzantium to gain ownership of Bosnia.
Serbia begins the Grand Campaign with no allies. From the first day, Serbia can be invited to Byzantium's otherwise worthless alliance with Trebizond. It is important to have Serbia in Byzantium's alliance before the Ottoman Empire has had a chance to declare war on Byzantium. Once Ottoman Empire declares war, a later invitation to alliance entangles Serbia in war against Ottoman Empire. Having Serbia involved in the war against Ottoman Empire is not ideal because Serbia is then distracted from war with the intended targets and Ottoman Empire may likely disrupt plans of attack by invading Serbia's lands. For these reasons, Serbia should not be invited to war with Ottoman Empire. If Serbia declines Byzantium's first alliance proposal, a second and third attempt may be made; but this may make things more complicated if there is war with Ottoman Empire.
Starting the Game
If, due to the difficulty level, Byzantium begins with 100d or more, promote a tax collector in Thrace. Invite Serbia to Alliance. Reduce military maintenance for the army and navy to 50%; you need your money to last as long as possible. Increase Catholic tolerance to 100% and lower Orthodox tolerance to 50% for reasons to be explained later. Adjust the domestic policy slider and investment sliders to suit your strategy for later in the game. A move toward greater Aristocracy is good to eventually improve badboy reduction and diplomatic ability; full Aristocracy is needed for any improvement. Note that Serbia is not as likely to join Byzantium's alliance if stability has already been reduced due to adjusting the DP slider.
If Ottoman Empire declares war, do not invite Serbia to war if Serbia is in Byzantium's alliance. If Serbia is not in alliance, invite Trebizond to the war; Trebizond declines and leaves Byzantium able to join Wallachia's alliance if Serbia should happen to also join that alliance. In the early war with Ottoman Empire, mint the synchronized loot in either January or February of 1419. Next, promote a tax collector in Thrace or build three thousand cavalry in Thrace; those cavalry should retreat if they get into battle. It is best to use just over 1K infantry to loot Anatolia; cavalry is too valuable to risk losing but is required for looting in January. Any tiny amount of infantry or cavalry can be used to loot Smyrna; just retreat from battle. Split and reorganize can be used to create tiny armies. It takes exactly one month to cross the strait separating Thrace from Anatolia and Smyrna. Divide the Byzantine fleet up into small numbers of ships and send all ships except one galley to the Aegean Sea to prevent Ottoman Empire from blocking the strait crossing; each separate fleet has an equal chance of engaging the enemy. Leave one galley in the Sea of Marmara to block Ottoman Empire from crossing the strait. Once looting is accomplished, retreat from naval battle. Keep ships in Sea of Marmara or in port; only fight naval battles where Byzantium greatly outnumbers the enemy. If Serbia is in an early alliance, have mostly all troops present in Bulgaria by April 1, 1419 in preparation for war with Ragusa.
If Ottoman Empire does not declare war, send four galleys in four separate fleets directly to the Adriatic Sea just outside of Ragusa's port. Load all troops onto remaining ships and send them to the Straits of Ontranto, making port in Morea. If Serbia is in an early alliance, Byzantium's ships loaded with troops should arrive in Straits of Ontranto by May 1, 1419. The other ships should arrive in the Adriatic Sea shortly thereafter in order to delay Ragusa's ships from arriving in Straits of Ontranto.
If Bosnia and Ragusa are allied, declare war on Bosnia; otherwise, declare war on Ragusa. If troops move by land, declare war as soon as a diplomat is available and immediately invite Serbia to the war. If troops move by sea, declare war on the fifth of the month when there is a diplomat available and invite Serbia to the war once separated cavalry and infantry have unloaded to Ragusa; retreat the initial battles against Ragusa in this case. Following diplomats should be used to make and break royal marriages with Hungary.
The war against Bosnia and Ragusa may go a number of ways. Minting and looting Bosnia and Ragusa is advised. If an early attack on Ragusa fails, collect men in Kosovo. Remember to anticipate Serbia's attack on Bosnia if Bosnia is in the war. Otherwise anticipate Serbia's attack on Ragusa. Send three thousand infantry to attack where Serbia is expected to attack. If Serbia does not begin marching within a few days, then stop Byzantium's advance and send the men once more. The first country to arrive in battle will gain control of the siege if a battle is won. If Byzantium's men arrive late, they can retreat so as to give Byzantium another chance to control the siege. If Byzantium's men retreat before Serbia's men arrive, a split operation on the retreating men can gain Byzantium control of the siege. If more than 1200 men are in control of a siege, a cover force of approximately 1200 men may be broken off and control of the siege can be maintained. The 1200 men units may be moved; but the odd number of men must remain to control the siege. Do not reduce the men in control of the siege below a few hundred; otherwise the men may perish from attrition before the siege is complete.
If Bosnia is not in the first war, wait until peace is made with Ragusa before declaring war against Bosnia. If Bosnia and Ragusa ally, a second war with Ragusa may be had when declaring war on Bosnia.
Peace in the Initial Wars
The goal is to complete sieges in Ragusa and Bosnia in the name of Byzantium. The most important and urgent goal is to gain control of Ragusa. If there remains a war against Ottoman Empire, do not take money from Ragusa until the abandoned war with Ottoman Empire has ended. Until there is peace with Ottoman Empire, try not to complete a siege on Ragusa if Serbia is the alliance leader, as is often the case when Byzantium must join Wallachia's alliance in order to be allied with Serbia. After about one year at war with Ottoman Empire, offer Morea for peace. The Ottomans will refuse; but the wheels of diplomacy will be greased. Soon Ottoman Empire may make her own demands for Morea and whatever money is in the treasury. Only accept peace once additional loot raises the treasury above what Ottoman Empire is demanding; otherwise a loan may be forced. Either Bosnia and Ragusa, or the western part of Ottoman Empire may be looted a second time before peace is made. If Ottoman Empire gains a 100% victory over Byzantium, it is only a short time before Ottoman Empire wipes out Byzantium's treasury. Byzantium had better have control of Bosnia or Ragusa at that time in order to be able to demand money and prevent a forced-loan.
Before making peace with Ragusa, it is best to have ships in Ragusa's port so that there is a place for Byzantium's men to avoid attrition on land. When making peace with Ragusa, a completely separate peace should be selected by clicking on Byzantium and Ragusa's shields in the peace proposal window. Take all of Ragusa's available cash and force-vassalize Ragusa; this will increase Byzantium's manpower and support limit somewhat, thereby reducing military support costs; there is a bug that gives a good deal more manpower for the first vassal. Serbia is unlikely to press a war with Ragusa so long as they are at war with Bosnia and Bosnia is not yet or is no longer controlled by Byzantium. Once money is taken from Ragusa, promote a tax collector in Thrace unless one is already promoted. Increase Byzantium's fleet to 20 galleys or more. If not at war with Bosnia, declare war on Bosnia immediately after making peace with Ragusa.
When making peace with Bosnia, immediately make a completely separate peace, taking Bosnia's cash and military access. After a matter of time Serbia begins its own siege of Bosnia. While Serbia besieges Bosnia, it is time to collect men where there is military access in Bosnia. This causes great attrition to Serbia's troops. Allow Serbia to maintain a siege force of five thousand men in Bosnia. Be sure that much of the Byzantine army is on the move to be able to arrive in Ragusa to avoid attrition when Serbia annexes Bosnia. Eventual war with Serbia is required to gain ownership of Bosnia.
If Serbia is near annexation of Bosnia, do not honor any alliance calls to additional wars. Otherwise there is one possible war of opportunity that may be profitable. While Serbia is at war with Bosnia and Ragusa, Albania may declare war. If Albania does declare war, there are two months to honor the alliance. Honor the alliance at a time when Albania's men are in Kosovo and there are over five thousand of Byzantium's men in Ragusa. If there is not yet a siege in Bosnia, retreat from any battle in Kosovo. If the battle in Ragusa fails, Byzantium's men will retreat to Albania, thereby covering the province and preventing the building of more troops. To press the attack on Ragusa, additional Byzantine troops may be required to pass through Bosnia or Bulgaria, Macedonia, and Albania. The goal of war with Albania is a completely separate peace taking money and forced-vassalization. War with Albania's alliance leader, Naples, likely results in white peace unless Naples has been greatly weakened due to other wars in Italy or unless Naples allies with Aragon. In either of the latter cases, more cavalry may be required to press a war on Naples home soil.
If Croatia declares war on Serbia, do not honor the alliance call, especially if Hungary joins the war against Serbia. It is not desirable to end up in an alliance with Serbia for too long. Hungary is too massive to fight; besides it can be advantageous if Hungary gains ownership of Kosovo. If Hungary declares war on Serbia, Byzantium especially does not want to be involved in war against such a giant.
Alliance with Hungary
Leave the alliance with Serbia as soon as possible. In order to reduce deteriorations in relations with Hungary, Catholic tolerance should be maximized or nearly so. Also, many royal marriages may be made and broken at a cost of insignificant stability reductions. Maintain royal marriages with Hungary as long as possible. These royal marriages should be formed during war with Bosnia and Ragusa. Remember that breaking a royal marriage costs two diplomats if two or more are in reserve but only costs one diplomat if there is only one available. Once relations are in the area of about -8 to +25, and money has been received from Ragusa, send one or more personal gifts to Hungary. The goal is to get relations with Hungary to significantly positive levels. Request to join Hungary's alliance. Next the goal is to bring relations with Hungary to +180 with gifts or royal marriages.
Wars with Serbia
At the time Bosnia is annexed, it is very possible that Hungary is already at war with Serbia. Otherwise, once Byzantium joins Hungary's alliance and Serbia has annexed Bosnia, Byzantium can declare war on Serbia. Invite Hungary to war with Serbia.
In war against Serbia, it is most important to gain control of Bosnia. If some of Byzantium's battles are won and Serbia's capital is controlled by Byzantium, Serbia will allow a separate peace giving Byzantium ownership of Bosnia. Likely, Hungary will take ownership of Kosovo at the conclusion of war with Serbia; do not take control of Kosovo since Hungary will likely give it to Byzantium. If Hungary owns Kosovo and Byzantium has military access through Hungary, then Byzantium has land access from Bosnia to attack Ottoman Empire in case the Turks should declare war. If Hungary is already at war with Serbia at the outset, then another country may gain Kosovo, which should cause no worries since Hungary can annex Serbia.
Once Hungary concludes the war against Serbia, it is time to for Byzantium to declare war on Serbia once more. The stability hit for breaking truce is of little importance; however the religious sliders should be balanced so that revolt risk is minimized in both Thrace and Bosnia. In order to prevent other allies of Hungary from honoring the alliance either on this war, insults should be dealt out to reduce relations with other allies to negative. In order to ever lead an alliance primarily formed by Hungary, it is important that Byzantium is the longest lasting ally of Hungary. Preventing other allies from honoring a call to the alliance also prevents them from being likely to annex Serbia. After all allies other than Hungary have dishonored this alliance call, improve relations with new allies of Hungary to positive levels, especially Wallachia and Moldavia. Hungary will likely annex Serbia thereby lowering relations with Byzantium and Hungary but preventing Ottoman Empire from easily annexing the province. After Serbia is annexed, Hungary is then able to attack Ottoman Empire directly.
War with Poland and Lithuania
It is possible that Hungary and Poland will already be at war. This is a war you would like to join. By this time, after 1424, Byzantium's Konstantinos XI should be on the scene. If Hungary is not at war with Poland and Poland is allied with Lithuania, declare war on Poland. Invite Hungary to war. If there is a small country such as Brandenburg that joins the war on Poland's side, then target Brandenburg for one or more easy victories against new recruits. Brandenburg may accept a quick and separate white peace, thereby arranging a separate war for Byzantium against Poland and Lithuania while still keeping Poland and Lithuania distracted with the war against Hungary. During this war, Poland may be looted but Lithuania is especially ripe for looting. Mint the loot to improve the war chest. At times, a few thousand more cavalry may be needed for the looting. Improve relations with Moldavia to +180; military access from Moldavia may help when looting Lithuania. Also, access through Moldavia can provide a short sailing distance for transporting troops between Bosnia and Thrace.
The war against Poland and Lithuania should be pursued for a while, perhaps something like five years. Konstantinos XI should claim all nearly completed sieges started by Hungary or other allies. The completed sieges help offset the war score lost from the many battles may be lost when looting Lithuania. Once Hungary concludes war with Poland, there may be one more looting season against Lithuania. However, shortly thereafter, peace should be made with Poland.
Wars with Ottoman Empire
It is possible that Ottoman Empire declares war on Byzantium in about 1425 or so. In that case, forget about the war with Poland and invite Hungary to war with the Turks. Proceed as below but with a smaller army.
After war with Poland, in approximately 1430, Byzantium should have several hundred ducats. Ottoman Empire's army should be reduced from wars elsewhere. Build up to about 40 galleys while waiting for Byzantium's war exhaustion to dissipate. If any other countries control any provinces of Ottoman Empire, wait for Ottoman Empire to end its wars. Build a few thousand more cavalry and again wait for war exhaustion to dissipate. A total of 20 thousand cavalry may work best. If Ottoman Empire still has a great number of troops, then make sure Hungary has positive relations with Byzantium and declare war on Ottoman Empire. Otherwise if Ottoman Empire is very weak and if Wallachia and Moldavia are in alliance with Hungary and Byzantium, send insults to Hungary to make relations negative while maintaining positive relations with Wallachia and Moldavia. Invite the alliance to war against Ottoman Empire. If Hungary dishonors the alliance, then Byzantium becomes leader of the alliance and Bosnia is safe from Turkish invasion. Otherwise, Hungary will deal a serious blow to the Turks. Either way, Konstantinos XI can claim sieges in the name of Byzantium.
If Byzantium is not the leader of the alliance against Ottoman Empire, then a separate peace may quickly be desired. Next Byzantium can break truce and declare a second war against a very weak Ottoman Empire that is also at war with Hungary plus other countries. Konstantinos XI can then claim more sieges and Byzantium can go on to gain a 100% peace against Ottoman Empire. Remember, in order for a 100% peace, all provinces must be controlled by Byzantium and every bit of the Ottoman's military must be destroyed including ships.