system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does Russia want to invade Ukraine?

I'd also like an explanation on why Ukrainians rebels want to fight to join Russia?
gpt
Crimea was conquered by the Russian Empire during the reign of Catharine the Great in 1783.  Russia fought two wars with the Ottoman Empire in Crimea, largely over the peninsula's strategic and economic value.  In 1954 the Soviet republic of Russia transfered administration of the Crimean peninsula to the Soviet republic of Ukraine.  
However Crimea remains predominantly ethnic Russians.  Ethnic Russians, both in Crimea and eastern Ukraine, are afraid of the new pro-Western Ukrainian nationalist government.  Combine that with pro-Russian propaganda coming from the east.  Russians in Crimea and eastern Ukraine want to be part of Russia again.  
Russia wants that territory back.  They ceded administrative control over an area with a great deal of strategic and economic value.  Now that area is a part of another, possible unfriendly, country.  With Ukraine's new pro-European government replacing their earlier pro-Russian government Russia no longer feels they can trust Ukraine with what they still consider their territory.  
Russia sees unhappy ethnic Russians in a territory they still consider rightly theirs which they want back.  So they support rebels that will either secede from Ukraine to rejoin Russia or act as counter-revolutionaries and restore the earlier pro-Russian government.  When Ukraine tries to stop the rebels Russia claims they are attacking ethnic Russians and steps up support for the rebels.