This is because the US government hoped that Russia would persuade then Ukrainian President Yanukovych not to use force to solve the anti-government demonstrations. However, Yanukovych was overthrown later, so the Russian government felt cheated by the United States, and the relationship with the United States changed greatly. And Ukraine is very important to Russia, because Ukraine is located between NATO and Russia. It can be said that Russia regards Ukraine as its back garden and its strategic living space.

The Ukrainian issue can be said to be a game between Russia and the United States. Although the civil war in Ukraine is an internal conflict in Ukraine on the surface, there is a shadow of a big country behind it. This is because people in eastern Ukraine are very close to Russia, while people in western Ukraine are closer to western countries. Therefore, the country of Ukraine is very divided, and some big countries behind it provoke disputes, which leads to fierce conflicts between the two sides.

Later, in order to protect its own interests, Russia chose to occupy Crimea. Because the Crimea region is very important to Russia, because once it is occupied, it is equivalent to safeguarding the interests of the Russian navy in the Black Sea region. And there are Russian military bases on the Crimean peninsula. Once strategic deterrent weapons are deployed in Crimea, it will seriously threaten the security of NATO countries.

So Russia later warned western countries not to act rashly by occupying Crimea, but it was also strongly sanctioned by western countries, so now Russia's economy has experienced a great decline.