XP

When picking a country to conquer, it is important to be aware of the state that the current civilization is in. If there has been an extended peaceful period and the civilization doesn't have enough enemies or external issues, they'll start destroying themselves by finding issues within everything. Even things such as gender or race can be grounds to start "pointless" fight. This is the main sign that a civilization is going to end soon and all you'll need to do is use a few insiders to exasperate the situation. The only way to prevent the outright downfall at that point would be to go to war with another country or face a humanity threatening disaster such as a beer virus.

The more profitable and go to method is however to just start replacing the upper echelons and working your way down. Eventually you'll have another country under your control.