Short answers, pulled from the story.
Ukraine severed all formal diplomatic ties with Russia on the 24th of February 2022, the day Russia launched its full-scale military invasion. President Zelenskyy announced the break immediately after the invasion began.
The Budapest Memorandum on Security Assurances, signed in 1994 at Patria Hall in the Budapest Convention Center, prohibited Russia, the United Kingdom, and the United States from threatening or using military force or economic coercion against Ukraine, except in self-defense or in accordance with the UN Charter. In exchange, Ukraine agreed to destroy its nuclear weapons and join the Treaty on the Non-Proliferation of Nuclear Weapons.
Ukraine inherited the third-largest nuclear stockpile in the world, including 130 UR-100N ICBMs with six warheads each, 46 RT-23 Molodets ICBMs with ten warheads apiece, and 33 heavy bombers, totaling roughly 1,700 warheads on Ukrainian territory. Ukraine transferred all Soviet-era strategic warheads to Russia by 1996.
The Holodomor was a man-made famine in the Ukrainian Soviet Socialist Republic in 1932-1933 that killed up to 7.5 million Ukrainians. On the 13th of January 2010, the Kyiv Appellate Court posthumously found Stalin, Kaganovich, Molotov, and Ukrainian Soviet leaders Kosior and Chubar guilty of genocide against Ukrainians during the famine.
On the 27th of February 2014, Russian military men in masks with no insignia seized the Crimean parliament building and two airports. Following a disputed local referendum in mid-March, Russia formally annexed the peninsula. On the 19th of March 2014, all Ukrainian armed forces were withdrawn from Crimea, having been besieged in their bases by unmarked soldiers.
Malaysia Airlines Flight 17 was shot down on the 17th of July 2014 by a Buk surface-to-air missile launched from pro-Russian separatist-controlled territory in Ukraine. All 283 passengers and 15 crew members were killed.