Allied unity — and, most profoundly, resolve — is key in convincing Russian President Vladimir Putin that the cost of invading Ukraine is too high to bear.

Ukraine also must be allowed to choose its own orientation. As a free and independent state, the nation cannot be forced into Russia's sphere of influence, which is the end game Putin's troop buildup gambit seems to seek.

So far, so good for those opposing Kremlin coercion. Unlike the recent allied dysfunction on display when President Joe Biden only minimally consulted with NATO nations as the U.S. sped for the exits in Afghanistan, the U.S. and its European partners seem aligned on Ukraine.

Biden represented that unified approach relatively well when he warned Putin in a videoconference last week of severe sanctions from the West if Russia, which already cleaved Crimea in 2014 and has aided and abetted separatists in portions of Eastern Ukraine in the years since, were to fully invade the region with the 100,000 or more troops it has amassed along the border.

Unlike the relatively tepid response to the Crimea crisis, these sanctions, which may include the cancellation of the Nord Stream 2 natural-gas pipeline to Germany, could deeply destabilize the Russian economy (and consequentially contract Putin's approval ratings among Russians, which appears to be a significant subtext to Putin's manufactured foreign crisis). NATO would also send more forces to Eastern Europe, and more military materiel to Ukraine to help repel Russian troops, which still might overwhelm the overmatched Ukrainian forces.

What Biden did not and should not pledge is deploying U.S. or NATO troops to directly fight the Russians; such a full-scale conflict between the top-two nuclear powers could become catastrophic. But the nuclear issue may just be heating up: On Monday a Russian deputy foreign minister said that his country could deploy intermediate-range nuclear missiles in Europe in response to its belief that NATO plans the same, a charge the alliance dismissed. (Such deployments were banned by a 1987 treaty that the U.S. pulled out of in 2019.)

Biden rightfully did not overpromise on Putin's demands, including legal guarantees that NATO would not expand eastward. That decision should be for Ukraine (and Georgia, for that matter) and NATO to negotiate. A 2008 agreement to do just that is still stalled, but according to public opinion polls in Ukraine the desire to do so has never been higher. And no wonder, given Russia's aggression that goes well beyond and before Crimea.

Although it's possible that Russia will attack, the likelihood is relatively low, John E. Herbst, the U.S. ambassador to Ukraine from 2003-2006, told an editorial writer. "I don't think he will invade because the risk to him of invasion is high in part because the U.S. response to this threat has been quick and strong," said Herbst, now the senior director of the Atlantic Council's Eurasia Center.

Herbst, however, added that Putin has already gained some concessions, including the Biden summit and a European security dialogue, as well as a few minor amendments to the National Defense Authorization Act that was the center of Capitol Hill debate last week.

It's in the U.S. interest, Herbst said, to stick to principles the international community has widely agreed to, including "the right of countries to choose their own domestic and foreign policies, their rights of sovereignty and territorial integrity." Acceding to the Kremlin's quest to get a veto (even though Ukraine and Georgia are nowhere near NATO membership) "is manifestly not in the U.S. interest."

On Sunday, the G-7 backed these principles with a strongly worded warning of "massive consequences" and "severe costs" should Russia invade. That's helpful, but Moscow will be watching for Western wavering. So too will Beijing regarding its bellicosity toward Taiwan.

Now is the time for democratic allies to prioritize diplomacy, but not sell out countries that have a right to choose their own futures.