In 1970, the Temptations asked "War, huh, yeah, what is it good for." 43 years earlier, the international community, still recovering from the devastation of the "Great" War, had already answered that question with "absolutely nothing."
That's right, lead by French Foreign Minister Aristide Briand and American Secretary of State Frank B. Kellogg, the world entered into a treaty that literally outlawed war. This wasn't a League of Nations Treaty as the US never joined the League. Nonetheless, most League Nations did sign on. Even Germany. As did the Soviet Union.
Did this treaty outlaw war. Yeah, pretty much. The operative clauses of the Treaty are pretty short and to the point:
Article 1: The Hight Contracting Parties solemnly declare in the names of their respective peoples that they condemn recourse to war for the solution of international controversies, and renounces it as an instrument of national policy in their relations with one another.
Article 2: The High Contracting Parties agree that the settlement or solution of all disputes or conflicts of whatever nature or of whatever origin they may be, which may arise among them, shall never be sought except by pacific means.
That's it. The rest is how the treaty can be ratified or adhered to, and when it comes into effect. You can read the whole treaty here.
What did this really mean? War was flat out clearly. You couldn't attack your neighbors no matter the provocation. But the part about renouncing it as part of foreign policy meant you couldn't even threaten it as part of negotiations. No more ultimatums "Give us what we want or this means war." I
It didn't even really hide behind pre-texts. The second clause was you got a problem, you negotiate it. That's it. No more "well he started it so I was just defending myself." This closed the loophole from the Hague Conventions of 1899 and 1907 which allowed for self-defense.
At Nuremberg this meant, no matter the excuses that Germany gave for starting all the wars of aggression, they violated this treaty. Regardless of all the others, this treaty was violated. Even if Germany felt threatened, even if Germany said they were attacked first. War was not an option.
Germany freely signed this treaty as one of the original signatory nations. This was not a treaty of duress where they had to sign it like the Treaty of Versailles. They chose to sign it. Doesn't matter it was signed by the Weimar Republic not the Third Reich. The Third Reich was the successor state to Weimar and remained bound by all treaties.
Wait, there's no enforcement mechanism. Not explicitly in the treaty itself. But international law, like any law, is enforceable in court. Nuremberg was a court of law. The Nazis were put on trial, with due process, including the right to counsel and the right to present a defense. The Tribunal was merely applying the law to the facts of the case just like any other case that comes to court.
Of all the treaties cited in the Indictment, this is the strongest. There is no question that from at least 1935 when they remilitarized the Rhineland that Germany was using war as an instrument of foreign policy.
Is this treaty still in effect? Why yes it is. It doesn't seem like it, but it has never been renounced by nations, nor superseded. Article 54 of the UN Convention waters it down a bit as it explicitly allows for self-defense. As a later Convention, it would modify the earlier treaty. But Kellogg-Briand remains in effect to this day.
What does it mean practically? If Putin is ever tried for seizing the Crimea and then attacking the rest of Ukraine, he could be tried for violating this treaty. Putin might not see Ukraine as a sovereign nation, but only a wayward part of the former USSR, but that's not the rest of the world's view. Ukraine was a sovereign nation that Putin waged aggressive war against.
Is Russia bound by this treaty though since it was signed by the USSR which doesn't exist anymore. Yes it does. Russia is the successor state of the former USSR. It inherited all the rights and responsibilities of the former Soviet Union. If it wants the seat reserved for the Soviet Union on the Security Council with veto rights, then it is stuck with the parts like no war too.