A diplomatic agreement is an ongoing relationship between two countries. As the name suggests, most diplomatic agreements are created via diplomatic actions, although some are created by in-game events (i.e., casus belli from random events). Diplomatic agreements usually provide benefits to both countries, although most of them are assymmetrical. Some diplomatic agreements have a specific time period they hold for; these have an expiration date.
You can view any country's diplomatic agreements via the diplomacy screen. When you select a country, its agreements are shown on the left panel of the diplomacy menu. Expiration dates are shown for truces. You can determine the expiration date for other agreements by mousing over the line to get a tooltip.
Specific instances of diplomatic agreements, and their duration, are listed here:
- truces (5 years)
- royal marriages (25 years)
- military alliance (10 years, but renewed any time any member is in a war)
- vassalization (no expiration)
- military access (no expiration)
- trade agreements (no expiration)
- trade embargo (no expiration)
- permanent casus belli (no expiration)
- temporary casus belli (variable depending on cause)
- warnings (20 years)
- guarantees (20 years)