I'm having trouble finding Russia's official reason for invading the Crimea region of Ukraine. If my memory serves me right, didn't they change the reason halfway through the invasion?
At the time, there was no reason given, for the simple fact that Russia wasn't openly doing it. The Russian troops being used to take over strategic points (including the Crimean parliament) in the Crimean region of Ukraine were unmarked, and referred to by the locals as "little green men".
The official Russian line at this time was that the little green men must be locals, and if their weapons were Russian, they must have stolen them. This remained the story until after a referendum under the military control of the Russian troops showed that the local Crimeans loved Russia and wanted to join.
On Thursday, when asked about the soldiers widely known as the green men, Putin acknowledged that they were Russian. Their presence had been necessary, he said, to keep order so that Crimeans could decide their future in a referendum.
“We didn’t want any tanks, any nationalist combat units or people with extreme views armed with automatic weapons,” he said. “Of course, Russian servicemen backed the Crimean self-defense forces.”