Whatever the coming days and weeks and months bring in Ukraine, the Russian annexing of Crimea demonstrates that Putin is an unstable 'leader'. All those pictues of a shirtless Putin wading up rivers and riding horses weren't just the Russian version of shots of Harper with kittens.
He's probably gambled that the European dependence on Russian oil and gas, and the global incentive not to mess with prices will make any response to Crimea token at best. For godsakes don't upset the oil markets. A judo expert like Putin might believe that is actually excellent leverage. Which I suppose it is in the short-term.
That said, in the short-term the rouble has declined in value and there's serious talk about punitive measures against wealthy and influential Russians and the like. Russia might even be thrown out of the G8. I don't know if Putin factored this in, but this could have consequences.
So how to respond?
The long-term is a different story because Russian military adventurism acts as a counter-incentive to engaging with Russia. I don't imagine any country wants to be dependent on Russian energy supplies if they can help it and now that Putin has confirmed himself to be certifiable, they might really be looking for ways out of that trap.
Perhaps it is possible to undermine Putin by converting to renewable energy across Europe and destroying demand for Russian energy, and therefore a significant source of revenue for Putin.
As long as Putin is in power, there is a real risk of a another arms race and Cold War, which means a greater risk of nuclear war. Unlike the Cold War, Russians are not united in support of their leaders and there's very clear opposition to Putin inside Russia. Russia isn't isolated like it was in those days either and globalisation has meant it is also depenedent on the rest of the world for its wellbeing.
Again, these are vulnerabilities that Putin maybe has not appreciated to the degree that he ought to.
The worst thing anyone might do is promote an international military response as the way to solve this. Putin likes fighting and lord knows how he react to a NATO response.
The only military response that might help is the utterly unthinkable prospect of dragging Russia into a protracted Ukraine guerrilla war until Russians themselves tire of it and the man who started it, like Afghanistan.