Factbox: What is Crimea and why does it matter?
(Reuters) - Ukraine's Crimean peninsula is at the center of the worst East-West standoff since the Cold War.
Crimea, which juts out into the Black Sea off southern Ukraine, was absorbed into the Russian empire along with most of ethnic Ukrainian territory by Catherine the Great in the 18th century. Russia's Black Sea naval base at Sevastopol was founded soon afterwards.
More than half a million people were killed in the Crimean War of 1853-56 between Russia and the Ottoman Empire, which was backed by Britain and France. The conflict reshaped Europe and paved the way for World War One.
In 1921, the peninsula, then populated mainly by Muslim Tatars, became part of the Soviet Union. The Tatars were deported en masse by Soviet leader Joseph Stalin at the end of World War Two for alleged collaboration with the Nazis.
Crimea became part of Russia within the Soviet Union until 1954, when it was handed to Ukraine, also a Soviet Republic, by Stalin's successor Nikita Khrushchev, a Ukrainian.
Since the collapse of the Soviet Union in 1991, there have been periodic political tussles between over its status between Moscow and Kiev.
Russia's Black Sea base in Sevastopol gives Moscow access to the Mediterranean. Ukraine's fleet, carved out of the same Soviet fleet as Russia's, is also based there.
Russia leases its base in Sevastopol from Ukraine and a new agreement designed to last until 2042 was agreed in 2010, in exchange for discounts on Russian gas supplies.
The Ukrainian government placed restrictions within the agreement and Russia has since upgraded its own Black Sea port of Novorossisysk to take naval vessels.
The mountainous peninsula is attached to the rest of Ukraine by a narrow strip of land in the north. To the east, it is separated from Russia by the narrow Kerch Strait. Russia plans to build a bridge across the strait.
With an area of 27,000 sq km (10,000 sq miles), it is slightly smaller than Belgium. It is Ukraine's only formally autonomous region, with Simferopol as its capital. Sevastopol has a separate status within Ukraine.
Around 2 million. Ukraine's 2001 census showed around 58 percent were ethnic Russian, 24 percent ethnic Ukrainian and 12 percent Tatars, who support the new pro-Western government in Kiev.
Crimea's temperate climate makes it a popular tourist destination for Ukrainians and Russians, especially Yalta, where the Soviet, U.S. and British victors of World War Two met in 1945 to discuss the future shape of Europe.
It accounts for three percent of Ukraine's gross domestic product, with 60 percent of its own output made up by services. The land is intensely farmed, with wheat, corn and sunflowers the main crops. Extra water supplies are brought by canal from Ukraine's Dnieper River.
There are chemical processing plants and iron ore is mined in Kerch. Ukraine has two grain terminals in Crimea - in Kerch and in Sevastopol. According to UkrAgroConsult, these have exported 1.6 million metric tons of grain so far this season or 6.6 pct of Ukraine's total exports.
(Sources: Reuters, State Statistics Committee of Ukraine, International Institute for Strategic Studies, Encyclopaedia Britannica)