Let's begin with when Crimea first became a part of Ukraine.
On 19 February 1954, the Presidium of the Supreme Soviet of the USSR issued a decree on the transfer of the Crimean region of the RSFSR to the Ukrainian SSR. This Supreme Soviet Decree states that this transfer was motivated by "the commonality of the economy, the proximity, and close economic and cultural relations between the Crimean region and the Ukrainian SSR".
basiclly, there was no vote, they gave the land away to Ukraine which is fine and all until you realise there live people there who may of didn't even want to become Ukrainian..
which leads us to->
On March 16, Crimea's government claimed that nearly 96% of those who voted in Crimea supported joining Russia. The vote received no international recognition and, aside from Russia, no country had sent official observers there.
On March 17, the Crimean parliament officially declared its independence from Ukraine and requested to join the Russian Federation.
which means that in a democratic point of view, this is fully legal since it is a (first ever?) vote without any government throwing the province neither back or forth without the people deciding.
Imagine if you lived in Amsterdam and the Netherlands' goverment decided to give away Amsterdam to Turkey which means you are now officially a turkish citizen without there being any vote. It is outrageous, no? the vote comes late, but came nevertheless with an entire 96% vote FOR russia, BY the crimean population.