A military move into eastern Ukraine would greatly increase the dangers of a political, military and economic blow-back sufficiently powerful to threaten the leadership in the Kremlin. Western military analysts have no doubt that, in the first instance, the Russian army would swiftly overwhelm Ukrainian forces. But recent history suggests that when the worlds leading powers resort to military intervention against a hostile local population they almost always suffer a long-term strategic defeat.
Putin, who has lamented that the collapse of the Soviet Union was the greatest "geopolitical disaster of the 20th century", should know that the "disaster" was greatly accelerated by the draining effects of the Soviet war in Afghanistan. Even the mighty US was unable to win in either Iraq or Afghanistan.
Obama has apparently learnt the lessons of these failed wars far better than Putin. Of course, the Russian (and US) public gets a certain retro thrill from a macho leader who is willing to send in the tanks. But, in time, they end up lamenting his folly.
Why is it that military force has become so much less effective in achieving political goals? Jeremy Shapiro of the Brookings Institution suggests that changes in military and social technology have made it much harder for invading armies to secure a lasting victory.
Unless the population of the area that has been invaded is tiny, an insurgency is likely to develop. Modern-day insurgents usually have weaponry that can inflict steady casualties.
They now also have social media and mobile communications technology that make it much easier to get organised. If the insurgency also has powerful external supporters, it can be almost impossible to subdue.
