The ongoing crisis in Ukraine came to a head when Russian troops rolled into the Crimea region of Ukraine on Saturday. The troops marched on several Ukrainian military bases and demanded that they surrender their weapons.
“This is a red alert. This is not a threat. This is actually a declaration of war to my country,” said Ukrainian interim Prime Minister Arseniy Yatsenyuk.
Ukraine has been in turmoil due to widespread protesting against government corruption which then turned violent, resulting in widespread violence. Ukrainian Prime Minister Viktor Yanukovych fled to Russia last week due to what he called the “pro-fascist” forces.
Russian President Vladimir Putin sites the danger to Russian citizens in Ukraine posed by the unstable political situation as the reason for mobilization. Fifty-eight percent of Crimeans identify themselves as ethnic Russian.
US Secretary of State John Kerry condemned Russia’s “invasion and occupation of Ukrainian territory.” Canada withdrew its ambassador to Moscow in protest, and France and the UK have suspended their attendance to the G8 summit in Sochi this June.  
Crimea is an autonomous republic located along the Black Sea in southern Ukraine. Its direct access to the Black Sea makes it an important trading depot.
Ukraine remains divided over the incursion, with many Ukrainians welcoming Russia’s presence. Ukrainian pro-interim government protester rallied in Kiev in response to Russia’s military deployment.
Ukraine has mobilized its troops and called its reserves in response to the Russian incursion. Though tensions remain high on both sides, shots have not yet been fired between Ukraine and Russia.
Update: The US estimates that there are around 6,000 Russian naval and ground forces in Crimea. A senior US administration official stated that the Russian military has “complete operation control” of the region.
It was not immediately clear that Russia was launching a military expedition in Ukraine. The soldiers did not bear any insignia and were not acknowledged by Putin until 13 hours after the incursion began.