On December 5, 1994, in Budapest, Russia, the United States, and the United Kingdom signed a memorandum that would change the history of Eastern Europe. Ukraine, which at the time of the USSR's dissolution possessed the world's third-largest nuclear arsenal, agreed to completely give up its nuclear weapons.
In exchange, the three signatory countries committed to respecting Ukraine's independence, sovereignty, and existing borders. In particular, Russia explicitly committed to not use force against Ukraine's territorial integrity or political independence.
This commitment was violated by Russia in February 2014 with the annexation of Crimea, and later with the full-scale invasion in February 2022.
The Budapest Memorandum is a fundamental document for understanding historical responsibilities in the Russian-Ukrainian conflict. Its violation also has consequences for nuclear non-proliferation: which country will ever give up nuclear weapons again after seeing what happened to Ukraine?