1. A truce ends a particular conflict for a short time, whereas a long-term treaty removes the grounds for future conflicts and thus produces lasting peace. Supposed treaties that are mere truces are beneath the dignity of kings.
2. States aren't property and can't be bought or sold. A state is more like a person than a thing, "a trunk with its own roots." If it could be bought, it would have no authority over people.
3. Standing armies (armies amassed in peacetime) encourage war and should be abolished. Mercenaries (soldiers for hire) are engaged in immoral work; soldiers should volunteer without pay in the event of foreign aggression.
4. A national credit system that borrows from other states, with a greater likelihood of bankrupting them than repaying them, encourages war and must be forbidden. States should ally themselves against any state that uses such a credit system (e.g. England).
5. No state has the right or authority to interfere with any other state's constitution or government.
6. War is, by definition, the violent striving of two states to reach agreement on a matter about which there has been no lawful ruling. During war, states should forbid the employment of tactics such as spying and assassinations, because "some confidence in the character of the enemy must remain even in the midst of war" if peace is to become possible. War of total extermination must be forbidden.
Threat of war, if not open hostility, is the natural state of human society. Only in a civil state can neighbors agree to treat each other peacefully.
Three Definitive Articles for Perpetual Peace
1. "The Civil Constitution of Every State Should Be Republican"
All men are free, equal, and dependent on common legislation. A republican constitution requires the citizens' consent to fight in a war and pay for a war. "Republican" means that the executive branch is separated from the legislative; the constitution is more likely to be republican if the number of rulers is small, ideally monarchical. In a democracy, violent revolution is inevitable because everyone wants to be king.
2. "The Law of Nations Shall be Founded on a Federation of Free States"
Without losing their distinct identities, states should be bound by a joint constitution similar to their own.
Even warring nations pay lip service to the idea of law, because each human retains the hope of lawful behavior. A league of nations would require states to resolve disputes before a tribunal.
A treaty of peace ends one war; a league of peace ends all war. The idea that there ought to be no war can only make sense if there is a league of nations to generate and enforce the idea.
3. "The Law of World Citizenship Shall Be Limited to Conditions of Universal Hospitality"
Everyone must have the right to temporarily visit any place on earth. Hostility to visitors is contrary to natural law. Universal hospitality is the only way to peace.
Immanuel Kant. Perpetual Peace: A Philosophical Sketch. (1795)
This summary was written in 2005, along with a series of other 500-word summaries of philosophy books, as an exercise in brevity.