"If the people of Crimea choose to return to Russian rule after the plebiscite then that should happen."
The plebiscite in Crimea has next to zero chance of being held fairly.
Why not?
(a) the Prime Minister and Government of the Autonomous Republic of Crimea (which does not include Sevastopol) were overthrown at gunpoint (after gunmen took control of the parliament in Simferopol) a couple of weeks ago: the new "Prime Minister" is the leader of a party (committed to union with Russia) that got 4% of the vote, and 3 out of 100 seats at the last election there. (In all no more then 15% of the votes were for parties that are either openly or ambigiously for reunion with Russia) - although when the population of Sevastopol is added to this the % might be expected to increase by a few percentage points.
(b) Crimea is effectively under armed Russian occupation. Even if Russia had a history of running free and fair elections (which, to state the obvious, it doesn't), this would not be any way to run a fair vote.
(c) Ukrainian TV stations have just been prevented from broadcasting to Crimea: so the main media is now Russian state media, which is putting out incredible, breathtakingly dishonest propaganda
(d) Calling a referendum at 10 days notice hardly allows preparation of an opposition case, even without the factors outlined in points (a), (b) and (c)
Russia will have trouble from the Crimean Tatars/.
I wouldn't rule out Ukraine restarting its nuclear weapons programme in a consequence of all this: it has the means and know-how, and it gave up its (ex-Soviet) weapons in 1994 on the understanding that in return, it's territorial integrity would be protected by Russia, the USA and UK.
Point 1, Putin's claims he's only trying to protect the Russian minority in the Crimea are invalid as in the Constitution of Ukraine - Title II
Human and Citizen Rights, Freedoms and Duties Article 21, 24 & 26 the Russian minorities are protected under the Constitution of Ukraine.
He is simply trying to pull the wool over everybody's eyes when he has in fact illegally invaded a country.
Point 2, under the Constitution of Ukraine - Title III Elections. Referendum, it is quite clear that in Article 73 "Alterations to the territory of Ukraine shall be resolved exclusively by the All-Ukrainian referendum" which brings us to Articles 69, 70 but most importantly 72 which quite clearly states "The All-Ukrainian referendum shall be called by the Verkhovna Rada of Ukraine or by the President of Ukraine in accordance with their powers determined by this Constitution.
The All-Ukrainian referendum shall be convened as a popular initiative at the request of at least three million citizens of Ukraine eligible to vote, provided that the signatures in favour of the referendum have been collected in at least two-thirds of the oblasts with at least 100,000 signatures gathered in each oblast"
This means the Constitution of Ukraine says in effect that any referendum for the Crimea to secede to Russia would be illegal UNLESS The All-Ukrainian referendum is convened meaning every Ukrainian citizen would have to vote to permit or to deny the Crimea to secede to Russia and there's no way the Crimea government which in itself is illegal can hold a legal referendum
In my view, the Ukraine has been invaded by Russia on the pretext of "protecting" it's Russian minority and the Ukraine should do everything to protect it's own territory even if it mean asking the US to help defend a sovereign country and to the point of the Ukraine putting its armed forces on full alert to evict the Russians invaders by force if necessary.
If nothing happens to defend the Ukraine, where will Russian stop? What's stopping them invading other Eastern European countries on the pretext of protecting Russian Minorities?