Under American pressure, the NATO alliance has already intervened in a European War: the civil wars that broke up the former Yugoslavia in the 1990s. The 1999 air war over Kosovo was the alliance's first conflict. It did not begin under Article 5, in which an attack on any alliance is considered an attack on all members. Instead, NATO launched the air war -- without regular ground troops -- as a "humanitarian war." President Clinton played the key role in rounding up the alliance, which never even got a stamp of approval from the United Nations.
The war was effective. The ground conflict ground on -- killing thousands and displacing 840,000 Kosovar refugees -- but Serb forces were subject to air attack. NATO himmed and hawed about rules of engagement and risks to fighter pilots yet allied air forces eventually unloaded 6,000 tons of munitions hitting strategic targets in Yugoslavia and inflicting severe economic and military damage. Shortly after the Milosevic regime collapsed.
It is nearly unthinkable that NATO would deploy ground forces. But NATO members on Ukraine's periphery need to be reassured. Air operations are popular politically because they don't risk large troop casualties like ground operations. The United States knows how to conduct these operations especially against old Warsaw Pact air forces. American combat abilities and resources are superior to Russian ones; the United States has twice the aircraft and has conducted decades of large-scale exercises to simulate high-intensity war with a Russian-style air force, namely Red Flag.
Ukraine could easily request an air intervention over its territory and there is nothing the Russians could do about it. Such an intervention would be argued to be defensive in nature. And killing Russian conscript troops would quickly undermine public support for Vladimir Putin, who is already unpopular. An air war alone would not force Putin to withdraw, by any means. But it might force him to the bargaining table. And frankly, after crumbling over Crimea, NATO needs to prove it is not a paper alliance just to continue to project military power. -R.