MUNICH – A sense of helplessness and dread hangs in the air over the Western leaders gathered here at the Munich Security Conference as the expectation grows that Russian President Vladimir Putin will unleash a military attack on Ukraine within days, if not hours.

Balanced against that is a renewed and reinvigorated sense of common cause and unity among the U.S. and its allies and partners, alongside an increased conviction of the historic moment. Not since the Cold War's end have NATO allies and their partners engaged in more intensive military planning, designing of sanctions, political consultation, and intelligence sharing. 

What's uncertain is what will be more defining for Europe's future: Putin's determination to reverse the Cold War's outcomes by recreating a Russian sphere of influence by force, or the momentary return to a Western common cause that it has provoked. In every crisis lies opportunity, but it's anyone's bet how deep the crisis Putin unleashes will be, or how lasting the Western response.

U.S. and Europe leaders alike have been hard-pressed to rally their citizens around the dangers Putin poses to post-Cold War principles: that borders can't be erased by force, that great powers can't be allowed to subjugate their neighbors, and that independent countries should be free to make sovereign choices about their alliances and associations. 

What has shifted the mood here regarding Putin's intentions to one of greater alarm over the three days is a mounting and indisputable tide of evidence that Putin is poised to launch the biggest military action the world has seen since 1945.

One U.S. official, with access to real-time intelligence, told me, "One can't reach any other conclusion from the growing evidence we see that Putin just wouldn't go to this level of trouble, cost and logistical gymnastics if he weren't intending to do something very serious with it."

The mood here is one of disbelief that such a conflict can be possible in modern Europe, after several years of focusing more on less kinetic issues such as climate change and pandemic response.

There is also a mood of resignation that all the West's threats of political and economic sanctions – and commitment to move NATO forces forward to allied countries on the eastern front should Putin further attack Ukraine — won't be enough to sway the Russian leader from what he considers his historic imperative.

Munich is awash with armchair psychologists, many of whom have met with Putin over the years, wondering why the preternaturally calculating Putin is rolling the dice now. He himself has told Russian colleagues he is entering "unchartered territory," and European officials who know him best believe controlling Ukraine has become more an obsession than strategy, some 22 years into power and shortly before turning 70. To restore what he called "ancien Rus" in his essay on Ukraine last summer, which would cement his place in his nation's history, regaining control of Ukraine alongside Belarus is non-negotiable.

European officials here give great credit to U.S. President Joe Biden's administration for preventing Putin from controlling the narrative by releasing intelligence, both open source and classified, regarding Russia's unprecedented troop buildup and plans for false flag operations intended to prove that Ukraine was provoking Russia's military actions. Within hours, U.S. officials also rebutted Putin's claims that Russian troops were withdrawing.  

Speaking here, Michael Carpenter, the U.S. Ambassador to the Organization for Security and Cooperation in Europe, said Russia has now deployed between 169,000 and 190,000 military personnel near Ukraine and in Crimea – far more than U.S. allies had known -- a disturbing increase from a force of 100,000 on Jan. 30.

Said Carpenter, "This is the most significant military mobilization since World War II." How, when and in what numbers Putin will use all these troops remains uncertain, but only a dwindling number of experts believe he won't use them at all.

General David Petraeus, former U.S. army commander and CIA director, told a lunch here on the Ukraine issue, that what's most revealing is the considerable deployment of "enablers" for combat that aren't usually present for military maneuvers. "You don't need field hospitals for exercises," he said. "You need them for invasion."

What's disturbing is knowing how long ago the West could have countered Putin's revanchism as he signaled this path 15 years ago here in a speech that landed in the conference hall of the Bayerischer Hof hotel like a hand grenade.

Several weeks later in April 2007, Russia launched a series of cyberattacks on Estonia, it invaded Georgia in 2008, it annexed Crimea in 2014 and then it backed Russian-separatists in the Donetsk and Luhansk regions of Ukraine. Along the way, Putin more brutally repressed opposition at home, while Russia was connected abroad to assassinations, poisoning, cyberattacks, election meddling and disinformation campaigns.

With a smile toward his Munich audience in February 2007, Putin said, "This conference's format will allow me to say what I really think about international security problems. And if my comments seem unduly polemical…then I would ask you not to get angry with me. After all, this is only a conference."

He got to his point quickly: "One state and, of course, first and foremost the United States, has overstepped its national borders in every way. This is visible in the economic, political, cultural, and educational policies it imposes on other nations. Well, who likes this? Who is happy about this?"

The height of audacity came when he quoted former U.S. President Franklin Delano Roosevelt on why the post-Cold War security order could not stand. "When peace has been broken anywhere, the peace of all countries is in danger," said Putin in quoting FDR.

Now, it is Putin who is breaking the peace.

There is another well-known historic association with this city and that is the Munich Agreement of Sept. 30, 1938, when Germany, the United Kingdom, France, and Italy ceded to Hitler the German-speaking Sudetenland of Czechoslovakia. At the time, Europe celebrated the agreement as a way to prevent major war.

The lesson of Munich then — Munich of 15 years ago, and Munich today — is the same: Appeasement doesn't reduce dangers but only increases them. Putin is unlikely to back away from his designs on Ukraine, but the U.S. and its partners can leverage the threat by sustaining their newfound common cause, in the face of the boldest assault yet on the post-World War II international order. 

—Frederick Kempe is the President and Chief Executive Officer of the Atlantic Council.