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If you assassinate someone and are caught then relations drop dramatially with that country, then they'll execute your diplomat and you'll get a casus belli.

You could also encourage disloyalty in their governors; drop the relation enough and you'll be able to declare war without consequences.
 
There are two lines of thought with regards to the assassination method.

The first one is that you simply send a dud character to try and assassinate any enemy character. He's hopeless, so has next to no chance of succeeding and is almost sure to be caught and either executed or imprisoned. Both of those results are valid causes for war, and losing a useless character can only be beneficial for your country.

The second is that you send a character with high finesse but not much else to kill a strong enemy general or a king whose heir is extremely unpopular. Finesse is what determines whether an intrigue mission is successful or not, so this character would have a decent chance of killing the target. The benefit of this is that even if you don't get your CB, you get to kill an important enemy character which might make your subsequent war easier to conduct.

@Borne2Run: even having a -200 relation doesn't remove the stability penalty for declaring war with no CB. The only thing that does that is having a Civilisation value in your capital more than 20 points above that of the enemy. Which is great for Rome, who starts with 90% Civ...
 
If another country keeps my POW generals as prisoners for years and years, is there anything I can do?
Can I offer to pay ransom? Can I negotiate somehow? Would they initiate anything?

Guess, I don't get a CB for them keeping my generals, do I?

Sending someone to free them would be cool...
(Sorry for necro-ing...)
 
If another country keeps my POW generals as prisoners for years and years, is there anything I can do?
Can I offer to pay ransom? Can I negotiate somehow? Would they initiate anything?

Guess, I don't get a CB for them keeping my generals, do I?

Sending someone to free them would be cool...
(Sorry for necro-ing...)

You should be able to negotiate their release for some gold (If you are monarchy). If you are republic then your senate has to agree with the negotiations though.