Actually, the whole country was under Russia as far back as the 18th century. Before that, it had been (for about 200 years) part of the Ottoman Empire.
I’m certainly no supporter of either Stalin or Putin, but there’s much more of a backstory than you say.
Also, there’s good reason not to trust the Tatars, who carried out slave raids in the Crimea as late as the mid-18th century. The (European) slaves were sold to the Turks.
Even as recently as yesterday, the loony Dagestan warlord came forward and offered his “services” to the Tatars in Crimea. So it’s a very complicated situation.
And looking at an ethnolinguistic map of Russia, Putin would be wise not to open a Wilsonian self determination can of worms, or Russia could find itself reduced to “Muscovy”.
The bottom line is Russia violated a neighbors territory with military force. period.