Early in the Cuban Missile Crisis, President John F. Kennedy mused to his brother and other members of the Ex-Comm that the presence of American Jupiter missiles in Turkey, and the subsequent shift in the balance of world power that created, had probably triggered the rapid Soviet up-arming 90 miles to the South of Miami.
McNamara would later say, adroitly, that the Cuban Missile Crisis was a domestic political problem for Kennedy. The challenge: either convince Americans that Cuban missiles were not any more of a threat than the Jupiter missiles were to Russia, or agree to get rid of the irritant — the Jupiter missiles — in the first place. The latter course is exactly the one Kennedy arrived at, with no thanks to his trigger-happy advisers.
How does the world extricate itself from a vehicle that looks like it's leading to war? Look at the domestic politics of Ukraine. The endgame here, and the functional goal of the United States and NATO, is NOT to pull Ukraine across the line into an insoluble economic and political relationship with Europe. Indeed, NATO membership for Ukraine has never been on the table. It would alienate Russia far too much and would be, frankly, unnecessary. Ukrainian independence, an ability to choose freely between Russia and the West, is a more sustainable status quo.
The overthrow of the government might have been Putin's casus belli, but it does mean that NATO will need to offer, along with carrots to Ukraine, the threat of a stick to Russia. As Christopher Dickey suggests, it might come in the form of a security guarantee, one that promises to defend the Ukrainian right to self-determination over the long-term. The long-term is key: This crisis will simmer for awhile. A process that allows Putin to claim that he successfully defended the Russian minorities in Ukraine without going to war, along with one that recognizes a future government legitimized by future elections (subject to UN or OSCE monitoring), is the proximate endgame. (I hope).
Both the West and Russia are going to have satisfy the demands of domestic politics. And NATO has a vested interest in actually mobilizing against Russia, for once, even if it does not intend to fight. Even if Putin knows that the West would never invade, a larger, well-coordinated NATO force, with weapons hot and red lines established, is simply not something he can easily ignore. Indeed, a NATO mobilization would give him the pretext to endorse or propose a process of sorts.