Crimean Parliament, surrounded by Russian troops, sets referrendum to secede from Ukraine and join Russia
Nobody, of course, consulted the Ukraine. Can you say "armed conquest?"
When Saddam Hussein did the same thing in Kuwait Putin is doing in the Crimea, we went to war.
Obviously, even post-Soviet Russia isn't Iraq, and there's little that we can do in the short term of a military nature without assets in the area sufficient to matter. But as the First Bush said of the precedent, "This cannot stand."
No referendum taken under the present circumstances will be anything like fair or meaningful.
ADDENDUM: Craig Ferguson did a riff the other night about how he seemed to recall some guy with a mustache doing pretty much the same thing in the Sudetenland that Vlad is doing in the Crimea, even down to giving the protection of the ethnic Germans as an excuse. He had a referendum, too. As Craig asked, "How did that turn out?"