The U.S. and NATO were right to send a strong signal of support to Ukraine, which faces an alarming buildup of Russian troops along its eastern border. Hopefully it will convince the Kremlin that the cost of an incursion necessitates a diplomatic, not military, outcome of a crisis Moscow created.

Before the apparent beginning of at least a partial drawdown in forces announced on Thursday, Russia had added upward of 80,000 troops and a significant amount of materiel near the border in the biggest buildup since 2014, when it illegally cleaved Crimea and supported separatists in Eastern Ukraine. The grinding conflict has left about 14,000 dead, including civilians, and devolved into a stalemate that threatens to turn into a full-scale war.

Among Russia's justifications for its repeated violations of Ukraine's sovereignty is the need to protect Russian-speaking citizens. But they were not endangered in any way by Kiev, and it's Ukrainians who need the protection from predatory President Vladimir Putin, who continues to seek regional hegemony over independent nations that were once imprisoned in the Soviet Union.

"The U.S. stands firmly behind the sovereignty and the territorial integrity of Ukraine," Secretary of State Antony Blinken told Ukrainian Foreign Minister Dmytro Kuleba last week.

In a well-choreographed confab with NATO leaders, Blinken traveled to Brussels last week along with Secretary of Defense Lloyd Austin, who announced that the U.S. was not only canceling the troop drawdown in Europe announced by former President Donald Trump, it was adding about 500 personnel to Germany in a move that gives credence to President Joe Biden's pledge to strengthen the Western alliance. "These forces will strengthen deterrence and defense in Europe," Austin told the German defense minister. "They will augment our existing abilities to prevent conflict and, if necessary, fight and win."

Preventing conflict is best accomplished by sending an unambiguous signal to Putin that the West finds Russia's bellicosity unacceptable. That message was echoed by NATO Secretary-General Jens Stoltenberg, who said last week, "Russia must end this military buildup in and around Ukraine, stop its provocations and de-escalate immediately."

Ukraine is not a NATO nation, although it clearly appears to want to join the alliance — and for good reason considering its vulnerability to Russia. Putin is vehemently opposed to that, but the decision should be dictated by Ukraine and member countries — not by a meddling Moscow. Putin's aggression isn't only a test for Ukraine, but for the U.S., too. There are three possible reasons for the buildup, and they are not necessarily mutually exclusive, Ivo Daalder, former U.S. ambassador to NATO, told an editorial writer.

Russia might be "trying to intimidate the U.S. to see what the U.S. would do," said Daalder, who is now the president of the Chicago Council on Global Affairs. "To what extent does the U.S. have the capacity to put the allies on notice and toward a unified posture?" Daalder added that it could be "meant to be a show of force to take the measure of Joe Biden."

If Putin is indeed taking his measure, the president showed his mettle with the April 15 imposition of more financial sanctions and the expulsion of 10 Russian diplomats in a response to election meddling and the "Solar Winds" hack of government and corporate computers, among other issues. And the administration has been clear and consistent that the stakes will be raised if opposition leader Alexei Navalny dies in Russian custody.

Secondly, Putin "is clearly under domestic political pressure; his popularity is waning, the whole Navalny thing shows their nervousness, a sense of weakness rather than a sense of strength," Daalder said. "All regimes, but the Russians in particular, tend to try to distract from domestic political purposes by enhancing a sense of threat and foreign adventurism."

This scenario seemed to play out on Wednesday, when Putin threatened an "asymmetrical, quick and tough" response if undefined "red lines" were crossed by Western nations, and scores of Russian citizens gathered across from the Kremlin to push for Navalny's release and push back against the Russian president's policies.

A third option, however, still remains: Russia "may have decided to take the next step in Ukraine; potentially take control of the Donbas region and maybe even take it within Russia." The alacrity of this possibility was heightened with this week's release of satellite photos showing aircraft and other materiel moved to Crimea in a larger buildup than originally thought.

It's not only Moscow that will grade Biden on this test. Beijing will be watching, too, as it calculates its aggression regarding Taiwan and maritime claims in the East and South China Seas. Other leaders, adversaries and allies alike, also will be evaluating what the new U.S. president does.

So far, Biden is acting strategically by shoring up alliances and sending a consistent message, one that should be amplified by signaling to Russia that the cost of military action outweighs the benefits, and that diplomatic de-escalation is the best, indeed only, method to prevent a tragic miscalculation.