The Budapest Memorandum

The US, Russia, UK and Ukraine agreed to it in exchange for Ukraine giving up it's nuclear weapons. By invading Ukraine Russia has violated it. And the US, being a signatory, "provided guarantees of Ukraine's security and territorial integrity" therein. Zalensky wanted to ensure those guarantees but obviously its other signatories, including the US, are reneging on their promise. Just like Putin knew they would. So he knows he can just keep going and he will. He must be stopped and now, for later will be too late.

"The Budapest Memorandum, signed on December 5, 1994, by Britain, Russia, the United States and Ukraine, provided guarantees of Ukraine's security and territorial integrity in exchange for Kyiv's surrendering of nuclear weapons. The document entered into force after Ukraine signed the Treaty on the Non-Proliferation of nuclear weapons. Under this treaty, the signatory countries committed to respect the sovereignty of Ukraine, to refrain from the use of force against it."