For most of its history, the U.S. avoided what George Washington termed “entangling alliances.”

During World War II and the Cold War, the U.S. aided friendly states to prevent hostile powers from dominating Eurasia.

The collapse of communism eliminated the prospect of any nation controlling Europe and Asia. But NATO developed new roles to stay in business, expanding into a region highly sensitive to Russia.

The invasion of Crimea has triggered a cascade of demands for NATO, mostly meaning America, to act.

In March the administration undertook what Secretary of State John Kerry termed “concrete steps to reassure our NATO allies.” President Barack Obama detailed: “Today NATO planes patrol the skies over the Baltics, and we’ve reinforced our presence in Poland, and we’re prepared to do more.”

The Eastern Europeans aren’t satisfied. An unnamed former Latvian minister told the Economist: “We would like to see a few American squadrons here, boots on the ground, maybe even an aircraft carrier.” A gaggle of American policy advocates agreed.

Ian Brzezinski of the Atlantic Council urged NATO or the U.S. alone to arm Kiev with “anti-tank and anti-aircraft weapons.”

A number of analysts would make Ukraine an ally in everything but name. “Determine that any further assaults on Ukraine’s territorial integrity beyond Crimea represent a direct threat to NATO security and … will be met with a NATO response,” wrote Kurt Volker of the McCain Institute.

[pullquote]Europeans have no reason to change so long as Washington guarantees their security.[pullquote]

Charles Krauthammer suggested creating “a thin tripwire of NATO trainer/​advisers” to “establish a ring of protection at least around the core of western Ukraine.”

AEI’s Thomas Donnelly proposed “putting one brigade astride each of the two main roads” connecting Crimea to the Ukrainian mainland, “backed by U.S. aircraft and partnered with NATO and Ukrainian units.” Robert Spalding of the Council on Foreign Relations advocated deploying F‑22 fighters along “with an American promise to defend Ukrainian skies from attack.”

Sens. John McCain, R‑Ariz., and Lindsey Graham, R‑S.C., urged increasing “cooperation with, and support for, Ukraine, Georgia, Moldova and other non-NATO partners.”

Of course, more must be spent on the military. “The past half-decade has seen the U.S. defense budget fall victim to the budgetary axe,” Ilan Berman of the American Foreign Policy Council complained.

Yet America’s military spending is up 37 percent over the last two decades, while collective expenditures by NATO’s other 27 members are down by 3.4 percent. Overall the Europeans spend 1.6 percent of gross domestric product on the military. None match America’s 4.4 percent.

Today, most NATO members continue to cut outlays. The Eastern Europeans, with the exception of Poland, have been reducing their spending as well.

Of course, U.S. officials insist that Europe should do more. But the Europeans have no reason to change so long as Washington guarantees their security.

Despite Europe’s anemic military efforts, it still far outranges Russia.

In 2012, the other 27 NATO members spent $319 billion on their militaries, compared to $91 billion by Russia.

With a collective GDP more than eight times that of Russia, the Europeans could do far more if they desired.

The basic problem, noted Stephen Walt, is that “president after president simply assumed the pledges they were making would never have to be honored.” Obviously, an American threat to go to war may deter.

But history is replete with alliances that failed to prevent conflict and became transmission belts of war instead.

In fact, alliances can encourage conflict by emboldening weaker partners. For instance, in 2008 Georgia appeared to believe that Washington would back it against Russia. Offering military support to Ukraine could have a similar effect.

Washington should bar further NATO expansion. Over the longer term the U.S. should turn responsibility for Europe’s defense back to Europe.

Americans should sympathize with the Ukrainian people, who have been victimized by Moscow. But that does not warrant extending military support or security guarantees to Kiev.

Today, Washington could best protect itself outside of the transatlantic alliance.