https://www.cfr.org/backgrounder/ukraine-conflict-crossroads-europe-and-russia?gclid=EAIaIQobChMI9NPth-Ht9AIVMQPnCh2jHwDcEAAYASAAEgIvCfD_BwE

Former U.S. National Security Advisor Zbigniew Brzezinski, in early 1994 in Foreign Affairs, described a healthy and stable Ukraine as a critical counterweight to Russia and the lynchpin of what he advocated should be the new U.S. grand strategy after the Cold War. “It cannot be stressed strongly enough that without Ukraine, Russia ceases to be an empire, but with Ukraine suborned and then subordinated, Russia automatically becomes an empire,” he wrote.

In the months after Brzezinski’s article was published, the United States, the United Kingdom, and Russia pledged via the Budapest Referendum to respect Ukraine’s independence and sovereignty in return for it becoming a non-nuclear state.