Crimea is now part of Russia. The EU and the USA should understand this simple fact. Russia has acted illegally and in ways which offend the West, but Mr Putin now has the Crimea under his control.
Imposing some sanctions retrospectively in a fit of pique makes the West look weak and is a kind of self harm. It looks like someone trying to play cricket in a glasshouse. The sanctions are not going to get Crimea back in the fold of the Ukraine.
The West should believe in democracy. That means we should have worked with the Ukraine for a new election to find a President of the Ukraine who could speak for all and hold it together. That means we should have helped the Ukraine organise a legal referendum on the future of the Crimea so they can settle their own future. Instead the West allowed or encouraged the overthrow of an elected President without an early election to replace him, and condemned the referendum in the Crimea as illegal without having a positive way of letting the Crimea have its say.
In the UK defenders of the Union in the Union Parliament have given Scotland the right to a voice on their future, showing the EU how these things should be handled. Identity and loyalty are important emotions in politics. Unfortunately elsewhere in the EU there is a determined attempt to stifle popular opinion and distort or change loyalties. The EU and the Spanish state are against Catalonia having a vote on its future. The EU and the Italian state are against Venice and the Veneto having a vote on its future, though an unofficial one is currently underway. The EU and the Ukraine have been against the Crimea having a vote on its future, and partly as a result they have lost the Crimea thanks to Russian actions which the West condemns but cannot stop.
If the West thinks Mr Putin might be emboldened to do the same again elsewhere, then the West needs to make clear in advance their disapproval and take the diplomatic and defence actions necessary so next time they have some control over events. The rest is just huffing and puffing. The best defence against the splitting of nations is good democratic government in each state that can command the loyalty and agreement of its peoples. Seeking to suppress or deny the existence of demands for self government cannot be the right way ahead.