The principle of international law says: States should keep their treaties.

Why only “should”?

States are sovereign. No power stands above them. So they are in a state of nature toward each other. Their rights depend on the particular will of each state. So times when they keep treaties alternate with times when they break them.

No judge:

There is no judge between states. At most there are arbitrators and mediators, and even these only by chance.

Kant’s perpetual peace:

Kant thought of a league of states that settles every dispute. Then war would be impossible. But such a league requires that all states agree. This agreement again rests on their particular will. So it remains a matter of chance.

Summary: States should keep their treaties. But no power stands above the states. So international law remains an ought, and Kant’s league of peace also remains a matter of chance.