Ukraine contends that the March 16 referendum, in which Crimea voted to secede from Ukraine and join Russia, took place at gunpoint. That bit, however, is untrue. Neither the “little green men” nor uniformed Russian soldiers were present at polling stations during the vote. There was no armed pressure on Crimeans, not even on the indigenous Crimean Tatars, who didn’t back the secession and mostly abstained from voting, to show up or to fill in ballots a certain way. The presence of Russian troops played a role (more on this later), but it wasn’t what made the referendum illegitimate.
There is numerous evidence that the referendum was held under duress of Russian military intervention. The Venice Commission also stated that "the massive public presence of (para)military forces is not conducive to democratic decision making" and that "the circumstances did not allow the holding of a referendum in line with European democratic standards". http://www.venice.coe.int/webforms/documents/?pdf=CDL-AD(2014)002-e
UNGA Resolution 68/262 recalled that "the territory of
a State shall not be the object of acquisition by another State resulting from the
threat or use of force".
On 1 March 2014 Russia's upper house of parliament approved President Putin's request for Russian forces to be used in Ukraine. http://www.bbc.com/news/world-europe-26400035