As a result of the protests and upheaval in the Ukraine directed at the pro-Russian president in the Ukraine (see
regarding that upheaval) during the first weeks of February, after severe clashes with citizens in which many were killed by the President's security forces, the Russian President fled the capitol and was impeached by the Ukrainian parliament.
Russia announced that the ouster of their ally was a "coup," and would not recognize the interim government that was formed according to the Ukraine constitution after the impeachment.
Significant concern was voiced, particularly by US Secretary of State, John Kerry, and US President Barack Obama, warning that any military intervention would be met with severe consequences. But Vladimir Putin acted and was intent first on securing the Crimea (which until 1954 had been a part of Russia), where the Russian Black Sea Fleet is ported in the Russian Naval Base at Sevastopol.
Putin is no stranger to the Crimea. He, along with most Russian and before them, Soviet, leaders, and a lot of the Russian public view the Crimea as a vacation area. Putin visited the Crimea in 2012 on vacation and while there rode Harley Davidson motorcycles with a group that he met with there.
It Crimea was ceded to the Ukraine in 1954 by Nikita Khrushchev and has been a part of the Ukraine ever since. But many Russians believe that the Ukraine, which is heavily populated with ethnic Russians (as are the 3-4 southeastern provinces of the Ukraine), should still be a part of Russia.
Beginning on Thursday night, February 27, 2014, reports began coming in of uniformed, armed men taking up positions at Crimea government buildings and the two major airports in the Crimea. The one at the capitol of Crimea, Simferopol, and the other at the Naval Base in Sevastopol. By Friday morning, pictures of the troops began surfacing and they were clearly wearing Russian uniforms and gear, though they had no insignia or other identifying marks. As the day passed, Russian aircraft began landing at these airports with reports of troops disembarking the aircraft, and pictures of the Russian landing vessels at the Naval Base surfaced of them unloading armor and troops.
Throughout the day, these forces spread out in the Crimea. They established themselves along major highways, at road intersections, in major cities and at intersections within the capitol of Crimea.
As the United States leadership continued to warn against any intervention, Vladimir Putin had already intervened and was securing the Crimea.