"The collapse of the Soviet Union is the greatest geopolitical catastrophe of the 20th Century." --V. Putin, April 2005
In justifying the Russian invasion of Crimea, Putin uses the fact that the majority of the people living there are Russian-speaking. According to the Budapest Memorandum of 1994, the Russian Federation, together with the U.S. and Great Britain, are committed to:
1. "Respect the independence, sovereignty and existing borders of Ukraine... 2. Refrain from the threat or use of force against the territorial integrity or political independence of Ukraine... 3. Refrain from economic coercion designed to subordinate to their own interest the exercise by Ukraine of the rights inherent in its sovereignty and thus to secure advantages of any kind..."
Apparently, in this case, Putin subordinates considerations of international treaties and sovereignty to the interest of the Russian state in Crimea.
On the other hand, consider his rhetoric with respect to Chechnya and Ingushetia, two restive regions of the Russian Federation in the North Caucasus. Here Putin justifies his repression on the basis of national sovereignty, despite the fact that ethnically, neither region has many Russian speakers. Legality and consistency is not what Putin is about.
Since the time of Peter the Great, the Russian government, whether led by Tsars, first secretaries of the Communist Party, or Putin, have been consumed by conquering and absorbing their neighbors. Territorial and political expansion are apparently in the DNA of the Russian State. Russia has for centuries been a danger to its neighbors, and continues to be.
One might have thought that the rise of Hitler, Stalin, militant and racist Japan before World War II, and Mao would have ranked as great geopolitical catastrophes in the twentieth century. Each of these resulted in the deaths of millions of innocent civilians, which the dissolution of the Soviet Union did not. But deaths of innocent civilians is not what Putin cares about.