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The Wall Street Journal has a helpful explainer about the significance the Crimean Peninsula holds to Russians. Situated on the Black Sea, the city of Sevastopol is seen by Moscow as the optimal place to expand its influence into points beyond the Mediterranean. The Crimea also has a turbulent history: first captured by the Russians during the 18th century from the crumbling Ottoman Empire, the region later became the center of the appropriately titled Crimean War, in which British and French forces succeeded in thwarting Russia's growing influence. The 20th century wasn't any calmer, either — Ukraine gained control of the region in 1991.
Click on over to the Wall Street Journal for a full explainer of why Crimea is so important to Russia.