There are two broad reasons for Russia's invasion of Ukraine:

• the battle over Ukraine is a battle for influence and power so that Russia maintains its influence on former Soviet states, which is why 20% of Georgia is Russian-occupied, Crimea got invaded, and Transnistria has Russian influence. Many would argue that it is because of nationalism spirit, while others would say that it's just imperialism. It's all up to debate;
• Secondly, there are security reasons for which Putin urged NATO not to expand eastwards, which it refused.

But what exactly happens if NATO expands eastwards? Especially, what happens if Ukraine joins NATO, and how would it impact Russia's security reasons?