Today Vladimir Putin is almost universally condemned as an aggressor. However, I think a short look into the history of Crimea over the past century can help us put Russia's actions into perspective.
Prior to 1944, the Crimean Tatars and the Russians were the largest ethnic groups on the peninsula, each group comprising over 40% of the population - the rest being Ukrainians. The Crimean Tatars were a Turkic people, Islamic by faith. While they lived all over the peninsula, they were a majority of the population in the South.
Following the Bolshevik coup in November of 1917, the Tatars of Crimea declared their independence from Russia, proclaiming the Crimean People's Republic. Within weeks, the Bolsheviks quashed the uprising and put an end to the Tatar's dreams of independence.
Although Crimea was in the Russian SSR (Soviet Socialist Republic) and not the Ukrainian SSR, Crimean peasants had joined their Ukrainian counterparts in resisting collectivization. Stalin retaliated by burning cropland and otherwise prohibiting agricultural production, and prohibiting the transport of food into the Ukraine and the Crimea. The result was state created mass starvation resulting in the deaths of millions. In the Crimea, the Communists ensured that the Russians were fed while the Tatars starved. Over 100,000 Tatars starved to death, and many others were deported. By 1933, when Stalin was finally able to collectivize the farms and end the food embargo, the Crimean Tatar population was half of what it had been in 1917.
During World War II, the Nazis captured the Crimea. Tatars joined Russians in partisan units in the Crimean mountains, and the Nazis never succeeded in "pacifying" Crimea's mountainous regions. However, the Nazis also formed three Tatar Legions, and several Tatar leaders, no doubt angry over their people's persecution under Stalin, openly collaborated with the Nazis. When the Red Army "liberated" Crimea, the Tatars were doomed. On May 18, 1944, Stalin, at the urging of the infamous mass murderer Levrentiy Beria, head of the NKVD, predecessor of the KGB, ordered the deportation of all Crimean Tatars to the Uzbek SSR (now Uzbekistan). According to Simon Montefiore, Stalin: The Court of the Red Tsar (2004), 1.5 million Crimean Tatars were deported, most jammed into cattle cars and offered no food enroute. 530,000 Tatars died enroute or in the concentration camps that greeted their arrival.
Stalin now had a new "problem" on his hands. Who would fill the depopulated Crimea? One solution was to declare the Crimea a Homeland for the Jewish People. Yiddush actor Solomon Mikhoels and Yiddush poet Isaak Fefer - the latter of whom Montefiore describes as a secret NKVD agent - approached Foreign Minister Vyacheslav Molotov with this idea. Molotov's wife Polina, who was Jewish and who was becoming sympathetic to Zionism as a result of the Holocaust, urged her husband to support transforming Crimea into a Jewish homeland. Stalin not only nixed the proposal, but used it to undertake his anti-Semitic campaign during the last years and months of his life - a campaign that culminated in the Doctor's Plot, that, had Stalin's timely death not intervened, might have been followed by the mass expulsion of Soviet Jewry - those who had survived the Nazis - to concentration camps in Siberia.
Thanks to Stalin, post-war Crimea was populated overwhelmingly by Russians, according to Wikipedia, 58% of the present population is Russian and only 24% is Ukrainian. In 1954 Nikita Khrushchev, who was emerging victorious in the internal power struggle that followed Stalin's death, ordered the transfer of Crimea from the Russian SSR to the Ukrainian SSR. It is unclear why Khrushchev did this - it certainly did not reflect the ethnic makeup of Crimea. Khrushchev may have been motivated by his own mixed Russian-Ukrainian heritage, and may have felt a tinge of conscience for the role he had played in the mass murder of Ukrainians in the early 1930's. Khrushchev's action was little noted at the time - the SSR's, "Soviet Socialist Republics", were hardly republics, possessing no autonomy from the centralized Soviet state. But Khrushchev's little noted action in 1954 would have vast repercussions 60 years later.