For peace you must have more influence than in CivII. (Com on enemy sneak attack + meat my goernment an peace. Next turn all again.)

My government type and oponent reputaion should affect to my willingness of declaring war (Even behind my back) and signing peace.

first can you define what the problem is? is it a significant problem? how does your ideas fix that problem specifically? does your fix effect any other areas of the game? if it does effect another area does it upset game balance in those other areas? is there a simpler way to fix the problem? does your idea hurt gameplay? why out of all of the ideas does your fix belong on this list?

reputation needs a total revamping in civ3 and it needs to tie into declaring war and peace...you should have the option of declaring war without it effecting your reputation in some circumstances, and i think morale and police vales in SE should effect the willingness of your people to goto war...also they should see if your civ would benefit or lose from declaring war/accepting peace when they do the calculations, and the AI needs to only fight if it has a good reason to

Scrap the entire current system! Something like this would be better:

First you must declare war 1 turn prior to any attack or the war is considered a sneak attack. Either way, the party who is being attacked has the option of declaring war or suing for peace. Sue for peace would go through the Diplomacy channels and may fail.

Then the idea of alliance needs to be strengthend. If you are allied with a country you automatically declare war if they are attacked. And you must wait until the next round to revoke the treaty and cannot offer peace until the following round.

That said here's the real solution:

There needs to be a relations value (say +/-100 with 0 being neutral) assigned to every civ. War declared in support of allies would drop the relation value 50 points. War declared without provocation would drop the value by 100 and a sneak attack by 200.

So if a civ starts out with a very good relation (say 80) and declared an unprovoked war, your relation value with them would drop to -20 but a sneak attack would drop it to a full -100. It would be possible to maintain a positive relation value if you are only allied to other warring factions (similar to our relation to the Italian people in WWII).

Each type of goverment would have a 'peace factor'. From Anarchy at 0 to Democracy at 100. The odds of accepting peace would then be determined by:

peace factor + relation factor/100

So in the above examples using a Democracy:

Allied:
100 + 30 = 130% (must accept peace)

Unprovoked:
100 + (-20) = 80% (still a good chance since their relations were very good before)

Sneak Attack:
100 + (-100) = 0% (No way in He**)

Relations could improve 10 pts for each turn that no agressive action is taken and a sue for peace is offered up to the zero value. That way no matter what you've got a couple of cracks at the other Civ before your government forces you to accept peace.

Additional relationship improvement can be gained by peacefull trade, ect...

ß = (Gov + Dip)/200 > 0
chance for peace = 1 + Log ß

Your examples would then be:
Beligerant declared war in support of Ally: 1 + Log 0.65 = 0.81

Beligerant declared war without provokation: 1 + Log 0.4 = 0.60