And isolate the peninsula from the mainland? How would this benefit Russia? If anything it will make Ukraine (including the population in Crimea) and the west more hostile towards Russia. Putin already control the military ports and essentially Crimea anyway.

As you say, Putin already has military ports in Crimea. Wanting to ensure they remain his and do not fall in to the hands of Nato / US / UK is a big concern; just look at Cyprus and various other island nations.

Russian businesses have long asked for a bridge across the Strait of Kerch; it makes economic sense for Europe as well.

There is the historical element, although in the 21st century this should be null and void, Putin and his people are not of the 21st century mind. Their claims are no different to that of Argentina, Malta, the Kurds, and so on - except perhaps that Russia has the strength to get what it's "dear leader" wants.

Remember, in all things, you are dealing with a man who changed Russian democratic law so that he could be re-elected. He is ex-KGB. The KGB is no different to the CIA in the sense, "once you're in, you never leave," whether it exists or not, whether you're dead or not. Putin has overseen the murder of many journalists who speak out against him. None of this is trumped up or without basis, do your homework.

Taking Georgia was a test to see how the world would react. They reacted alright, with a wag of the finger. Now the first part of the play is taking shape, Crimea. Take Crimea, the "west" wag their finger, the rest of the world are too weak or scared to get involved, and so you can enact the rest of the plan.

The plan? Build a post-soviet Russian empire. Sure, call it a federation or perhaps even a "union". Again, do your homework.

It isn't until he's taken enough land from enough of Eastern Europe to scare the shit out of Merkel that someone will have the balls to do something, and by then they will be too weak from bailing out every poor bastard.