How much of a threat to global stability is Vladimir Putin? How should not just the US but Europe respond to his threat?

As a nuclear power, Putin and Russia is as much a threat as any other country looking to expand its interests. While Putin May have been foolish annexing Crimea and attempts with the Ukraine, the West is just as foolish making overtures to former Soviet satellites to join either NATO or the EU. There is no doubt that Putin is aggressive in defense and pursuit of his country's interests, but that is why it is incumbent on the West not to inflame tensions by trying to influence the direction of former Soviet territories. Neither side trusts the other, which is why both sides need to mind their own business.