First of all, stop making treaties after wars with provisions for future wars. That's such a fucking copout. If you want everyone to stop fighting, just agree to stop fighting; But it doesn't count if you just say, "Yeah, fine, we'll leave them alone. Unless, you know, they start doing stuff we don't like or whatever. Then it's back on like Donkey Kong." You're that asshole on the playground who told the teacher, "Fine, I'll stop beating his ass. Unless he drinks his milk too loudly. Then he's fucked." That doesn't fly.
Second, stop trading countries, you imperialist fucks. States aren't things you can trade, any more than people are. Governments get to rule when they have a contract with the people; when governments buy and sell countries, they're just buying and selling people en masse. That shit is unacceptable; you deny the people the right to be ruled by their own government. Cut it out.