On March 18th, 2014, the Crimean peninsula, under Ukrainian law, seceded, illegally, from Ukraine, and, after a short-lived bout of independence, united with Russia. Obama has claimed that what occurred in Ukraine is a flagrant breach of international law, and insisted that we must respect the Ukrainian people's choice in their political futures.
What occurred there is not a breach of international law, except for the gas price hikes, as no Russian troops, officially, took part in the seizure of that peninsula.
The referendum there may have been illegal under Ukrainian law, but falls under the UN's jurisdiction, as they are a separate people, from Ukrainians, when it comes to self-determination, and they say they have the right to do so, along with other ethnic peoples.