The Crimea is not pro-Russian. The "Prime Minister" was appointed at gunpoint. Just because a slight majority of the people are of ethnic Russian origin does not mean that they support being invaded and annexed by Russia. If your reasoning was correct, America could be invaded and annexed by England, and you would approve.
Iran: IAEA visit to U.S. Senate damages UN neutrality (DPA)
from the article: Report: Russian soldiers storm Ukrainian military base in Crimea