system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What does Russia have to gain from taking over Crimea?

I understand Russia's only warm water port is located there, and about half the population are essentially Russian, but what do Russia and Putin have to truly gain from taking over Crimea? It seems like there's a lot more to lose than gain.
gpt
Crimea was a 19th century strategic necessity for Russia.  It provided Russia with an effective navy in the Black and Mediterranean seas when those waterways were critical to Russia's economy and Russia faced European powers that might have tried to strangle its economy during wartime.

Crimea today is a feint designed to allow Russia to test the resistance of Europe and America to its imperial intentions in Ukraine.  Most Russians consider Ukraine a part of Russia.  The very seat of the ancient Rus people, for whom the whole country is named, is in Kiyev.  Russian Maximum Leader Putin famously told President Bush that "Ukraine isn't even a nation".  Ukraine has some of the most fruitful farmlands in Europe.  The Ukrainian capacity for agriculture was a major point of Hitler's desires to destroy the Soviet Union - with Ukraine under German control, the Third Reich would have had food security and not be reliant on other countries for sustenance.  

Russia clearly sees Ukraine in the same light.  Russia + Ukraine is a much more powerful force than Russia - Ukraine.  In fact, Russia - Ukraine may not even be able to sustain itself as a significant power, becoming more akin to a France or a UK - requiring alliances and collective action to really make a difference in world affairs.

Russia has played a long game in Ukraine since the breakup of the USSR.  It has worked assiduously behind the scenes to keep Ukraine from doing the two things it thinks would irretrievably sever the Ukrainians from the Russians:  Joining NATO, and joining the European Union.  So far, Russia's interest in keeping Ukraine out of those entities has outweighed the desire of the Europeans and the Americans to get Ukraine into them.  Everyone knows that either step would force a serious confrontation with Russia, and nobody is sure if Russia would blink and back down, or suicidally drag everyone down into the abyss rather than relinquish dreams of being a global superpower.

So buckle up, kiddies.  This is one of the Big Ones.  History is being made right now.