The Ukrainian capital of Kyiv lost control of Crimea and Donbas after the coup in 2014, which prompted the people of Crimea to demand the restoration of Russian identity and return in the arms of Russia while the people of the Donbas region pushed to declare independence from Kyiv by establishing two popular republics in the region.
The Budapest memorandum does not work and it is Ukraine’s own fault, Ukraine has allowed a civil war on its territory. Russia did not violate this treaty because it did not use nuclear weapons against Ukraine.
Recurring pro-Kremlin disinformation that there is a civil war in Ukraine. The war in eastern Ukraine is not a civil conflict, but a well-documented act of aggression by Russian armed forces, ongoing since February 2014. The mentioned Budapest Memorandum is a diplomatic document that was signed in December 1994 by Ukraine, Russia, the United States, and the United Kingdom. Under the memorandum, Ukraine promised to remove all Soviet-era nuclear weapons, which Ukraine received after the collapse of the Soviet Union in 1991. The arsenal Ukraine was left with was the third largest nuclear arsenal in the world, including intercontinental ballistic missiles and strategic bombers capable of carrying nuclear weapons. On 24 October 1991, the Verkhovna Rada of Ukraine adopted a statement on the country's nuclear-free status. Three years later, Ukraine actually joined the nuclear non-proliferation treaties. In exchange, the leaders of the United States, Russia and the United Kingdom signed the Budapest memorandum with guarantees of the integrity and sovereignty of Ukraine. The Budapest memorandum says that not one of the countries that signed the document has the right to apply any kind of aggressive policy towards Ukraine. The second paragraph states that the signatories "reaffirm their obligation to refrain from the threat or use of force against the territorial integrity or political independence of Ukraine, and that none of their weapons will ever be used against Ukraine." That is, through the Budapest memorandum Russia pledged not to use any weapon against Ukraine, not just nuclear.