Joseph, in 1994 the leaders of the US, UK, Russia, and Ukraine signed an agreement (the Budapest Memorandum) that stated that Ukraine would give up the nuclear weapons that the USSR had kept there. Here’s part of the agreement: “The Russian Federation, the United Kingdom of Great Britain and Northern Ireland and the United States of America reaffirm their commitment to Ukraine, in accordance with the principles of the Final Act of the Conference on Security and Cooperation in Europe, to respect the independence and sovereignty and the existing borders of Ukraine.”
When Russia invaded Ukraine and annexed Crimea, it broke this agreement.
BTW, Ukraine was not the first country in which Russia covertly supported a group of ethnic Russians who wanted to remake the borders of former Soviet republics. There’s also Transnistria (a breakaway from Moldova, and the only territory that still has a hammer and sickle on its flag), Abkhazia, and South Ossetia (both breakaways from Georgia).