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.
Third, and this one's a big one: get rid of your goddamn standing armies. No, don't start that "but we need a defense!" bullshit. If everyone gets rid of their armies, you don't need to defend yourself from anything. Besides, do you think other countries look at your army and think, "Hey, look, they can deploy battalions to anywhere in the world in 72 hours! Yeah, that's definitely a defensive force." Because if you do think that, you're. A fucking. Idiot. If everyone thinks you're preparing to invade, they start building armies too, and then you've got an arms race on your hands. Eventually someone runs out of money and decides they have to strike first or fall behind - I think we aaaall know how that story ends. Look, if you want a defense, then let your citizens volunteer and train, but the last thing we need if we want peace is a military just standing around with bombs and shit, twiddling its collective thumbs and building more bombs.
Fourth, stop borrowing money to fuck with other countries. Governments can just keep borrowing and borrowing, because what could possibly be a safer investment than a government, right? Except nothing gets produced except more weapons; it turns into a big ol' Ponzi scheme where some people get paid back when new money comes in, and it only ends when the people run out of money to pay in taxes and the whole fucking thing collapses. And you know what's left when that happens? A bunch of pissed off people and a lot of extra bombs just lying around. So, hey, quit loaning each other money for shit like that, or else peace won't happen.
Fifth, quit getting your military involved in other governments and constitutions. Who the fuck gave you the right to send in "peacekeepers" or anyone else? The government is mistreating its people? Look, if a state falls into anarchy and splits in half, if there is no government, no constitution to speak of, then, you know, be helpful. That's fine. But until that happens, let the people have their independence, let them figure their own shit out. Maybe they don't want what you want. Maybe they want to make a government on their own, you know, as happens sometimes. And maybe it's not helpful for peace if, every time a government is accused of something, they have to look over their shoulder and see if the big bad powers-that-be are coming to punish them. You'd be surprised how competent other people are.
Sixth, if you absolutely must fight, fight honorably. Look, I get it. Sometimes assholes crop up into power and they start building a military, and trying to appease them doesn't work. So you have to go put on your business shoes and fuck them up so they'll stop invading other countries. I'm not saying don't do that. I'm saying do it right. If you use assassins and poison and shit, then no one will trust you, and rather than sign a treaty with a country they don't trust, they're going to keep fighting until someone get wiped out. And if that keeps happening, then sure, we'll have perpetual peace, but only in the vast fucking cemetery we've made for ourselves of this earth.
"But Kant, isn't that a bit unrealistic? I mean, how can we possibly make that all work?" Yeah, I'm not a fucking idiot. I know that bros love to fight each other in the state of nature, but eventually they got sick of that and figured out government. It turns out, governments are going to have to do the same thing. And before they do that, they need to be Republics. In a Republic, the power to make laws and the power to administer the laws are separated; anything else is despotism. When that happens, it's hard as shit to make war, since everyone has to agree to war, and everyone has to pay for it, and in that scenario people are real fucking cautious about getting involved, since war sucks for everyone. When you have the same people writing the laws as deciding what they mean, it doesn't matter if it's one person or all the people, shit goes down. What the fuck does a constitution matter, if the people it's supposed to bind get to decide what it means?
Once countries get their shit together and become Republics, they should band together and sign a constitution to form a league in the same way that people band together to form governments. You're a fool if you think that states don't have to follow laws outside themselves; whenever a state invades another, they always quote some philosopher or another on the "natural law" or some shit, but when you quote at them they're not like, "Oh, good point! Locke! Forgot about him. Yeah, I guess we'll stop this invasion now." They shrug and say, "Fuck that guy, he's wrong." Then they quote some other guy who agrees with this particular war, and go about laying waste to opponents. No one is saying, "This is obviously a completely illegal war, but we're doing it anyway." So it's not that countries don't follow laws, it's that they don't agree on which 'laws' to follow, and we've always let the victor decide after the fact. We need to put a stop to that shit, and the only way to do it is to agree on a law beforehand.
Now, I'm not saying we should form one huge world government, any more than people merge together to form one giant person. That's ridiculous - states get to be autonomous like persons are autonomous. All they're doing is agreeing to abide by certain laws; it's like a social contract of social contracts. It's just an alliance. Seriously, if you want peace, we have two ways to do it: either establish a law now via treaty and agree to hold each other accountable, or let a conqueror pick the laws after it's too late. "Neither" isn't an option.
Last thing: nothing we've talked about so far is philanthropy, like being a part of a treaty is some magnanimous gesture. We're talking about respecting people's rights here. I'm not telling you that you have to be charitable to have peace; seriously, just a baseline respect for human dignity will do. Just stay out of people's faces. After all, no matter what country you're a citizen of, you're a human just like the rest of us, a 'citizen' of the world. So when a stranger arrives in your land, just don't be a dick. We won't have peace until people stop assuming that everyone who is different is an enemy, someone to be feared or regarded with suspicion. You don't have to feed him or drink with him or even talk to him if you don't want to; you don't have to let him sleep in your house or anything. But for fuck's sake, let him chat with anyone who will chat with him, and unless he tries to harms you, just let him be. Otherwise, we'll always be different from each other, even though we're not. The better we understand each other, the less likely we are to go to war; ultimately, peace among nations must start with peace among men.
You can get the full text of the Perpetual Peace online, but if you prefer the print edition for some reason, try Perpetual Peace, and Other Essays on Politics, History, and Morals (HPC Classics Series), which includes Kant's supplemental writings in defense of this treatise.