The Russia-Ukraine Crisis Explained, checkout reasons.

Russia has attacked Ukraine. Here’s what you need to know about the crisis that could become Europe’s largest armed conflict since World War II.
The current conflict has severely strained U.S.-Russia relations and increased the risk of a wider European conflict. Tensions are likely to increase between Russia and neighboring NATO member countries that would likely involve the United States, due to alliance security commitments. Additionally, the conflict in Ukraine will have broader ramifications, specifically for U.S.-China relations, and for future cooperation on critical issues like arms control, cybersecurity, nuclear nonproliferation, energy security, counter-terrorism, and political solutions in Syria, Libya, and elsewhere.