How to get CBs

A casus belli (CB) is an internationally acceptable reason to declare war against another country. This way, DoW is a response to their hostile acts towards your country and not merely your desire to expand your dominions, something unacceptable to the international community. Therefore finding a CB against your target is very important in terms of minimizing your badboy increase, and keeping up your stability. There are two types of CBs. Permanent CBs do not have an expiration date, they disappear when certain conditions are no longer met. Temporary CBs have an expiration date that is shown in their listing on your country's Diplomacy screen. Let's review the reasons why you get a CB.

• Owning a country’s rightful possessions. Any country that has some of his core provinces in the hands of another country, has a permanent CB on them. Losing a province through defection gives the loser a 1 year CB on the receiver. Losing a province to Portugal or Spain due to the Treaty of Tordesillas also gives the loser a 1 year temporary CB that can be useful to start a colonial war.
• Annexing a country. Every country that annexes another one by force gives a two year CB to any country in the world whose relationship is < -150.
• Denying income. Every country that imposes a trade embargo another country gives a permanent CB to that country.
• Religion is the source of many CBs. A country changing its religion gives a two year CB to its former faith mates. Any Catholic country at war with Papal States gives a permanent CB to any other Catholic country. Owning Rome when you are not Papal States also gives a permanent CB to all Catholics. Becoming CrC gives a country a permanent CB on Protestants and Reformed until the Edict of Tolerance. Shi'ites have a permanent CB on Sunnis and vice-versa. Spain has a permanent CB on all pagans and Muslims.
• Dishonoring an ally's call to war. When an ally refuses to join in a war, all of that country's current allies receive a one-year CB against that country declining the war. The order of who dishonors first is relevant; once a country dishonors it does not get CB on countries that dishonor afterward because they are no longer allies. Except for a human player that makes the DoW, human players are last to decide if they want to join the war. This method of gaining CB has an added benefit, your actual intended enemy is left without allies; but you could wait for up to a year to see if the alliance comes back together if you want war with many countries. The AI sometimes dishonors an alliance call if relations with its ally are a good deal less than the country the ally is at war with. The AI never honors an alliance call if it has negative relations with the ally that made the call to war. The AI never honors if doing so would break a truce. The AI never honors if it does not know of the existence of the country which the ally is at war with, (i.e. the AI country does not know the enemy's capital and therefore has no relations with the enemy as can be seen in the relations screen). Most Europeans will not know about pagan countries and Europeans can DoW pagans without badboy; so get maps and then get an alliance with a country you want to DoW, DoW a pagan, make an alliance call, except for colonial countries your alliance members dishonor because they go, "Who is Aztecs?" The AI never honors if the war is over at the time of the alliance call. A human player can make the alliance call more than two months after the war begins; the war can be over by that time as is the case when many unfortified provinces are quickly captured on the first day of the war or soon after.
• Events. Scripted and random events can give CB on a country. Random events mostly only give CB on a country that you border.

If you are not provided a CB from your target's actions, there are still some diplomatic actions that you can try.

• Having a loan refused to be paid back. You might gain a CB from a country by offering a loan and having it accepted, and when it expires having it refused to be paid back. From 1.08, AI is much less likely to accept a loan proposal.
• Sending a warning. You might gain a CB from a country by sending it a warning. If that country attacks anyone during the next 20 years, you will get a 1 year temporary CB against it. Useful against aggressive countries in your vicinity. The AI ignores warnings.
• Proclaiming a guarantee. A more limited version of the warning, you guarantee the independence of any country of your choosing, so if any country DoWs your guaranteed country in the next 20 years it gives you a 1 year temporary CB. The AI is very fond of these and it is very funny to see Mantua guaranteeing the independence of China.
• Claiming the throne. You can claim the throne of a country with which you have a RM. It will cost you a wad of cash (the amount depends on the size of the target country), a stability hit of -3 and a fall of relationships of -100 with every country you have a RM with. For all that, it gives you a two year CB. Very few people use it except for a laugh or when managing the BB is a lot more important than stability, because you will also have to break the RM and might even get some more stability hits when DoW.
• Defender of the faith. Being the Defender of the Faith will cost you 1000d. That title gives you a CB against any country that gets in a war with any country of your religion, and a boost in troop morale. Reformed and CrC can claim the title, and the AI does it, but is a great way of blowing 1000d since they don't get the bonuses (bug). You will be the defender of your faith until you change your religion, or more commonly you lose a war. Losing a war means signing a tribute peace with anybody, even if only by 1D, or signing a white peace when you are the aggressor, even if you have beaten your enemy before getting tired of that war. For most people DotF is too expensive to be worth it, but you can have fun role-playing through the religion wars.

Now the question is how do you get that CB or DoW from your target country? First check your diplomatic info. You get CBs all the time through other countries behavior, but you are not informed about it. Also you can get a CB from a trade ban. If you don't have a trade agreement with your target, and flood his CoT with merchants, a country might ban you, although is not guaranteed (that CB appears at the beginning of the month after the ban). If you feel like burning 1000D you can proclaim defender of your faith (DoF, unless Reformed or Counter-Reformed Catholic), that will give you CBs against some of your heretic neighbours. Finally you can issue a guarantee on some of your target neighbours that he likes to beat. That guarantee is good for 5 years and the AI appears to be oblivious to them. Of course, if you join an alliance already at war with your target, you don't need a CB, and you don't lose your MA. You do lose your RM and take the -1 stab hit. If everything else fails you can always disarm and wait, although you can get the attention of some other unwelcome neighbor.