Why did Russia fight Ukraine?

Why did Russia fight Ukraine? Things can be traced back to eight years ago. On 20 14, Crimea, which originally belonged to Ukraine, decided to leave Ukraine to join Russia through a real-name referendum and officially belonged to Russian territory. Crimea is a strategic place in the Black Sea. The Crimean incident seems to be a perfect result for Russia, but Ukraine's national face has fallen sharply, and NATO, led by the United States, does not recognize it, but Crimea still insists on its own decision. This has officially become the beginning of the conflict between Russia and Ukraine. In addition, the fuse of the Russian-Ukrainian war was another independent region-Donbass, which was also recognized as formal independence by the Russian President. Because of the continuous division of Ukrainian territory, a large-scale armed conflict broke out between Ukrainian government forces and civilian armed forces. Finally, through the mediation of the international community and the signing of a ceasefire agreement, a temporary ceasefire was entered. Since U.S. President Biden took office, sparks have kept rising, and Ukraine wants to join NATO.