AFTER RUSSIAN democratic leader Boris Nemtsov was gunned down in Moscow last weekend, I posed this question at a Harvard Kennedy School conference: Is it possible that, in Vladimir Putin’s highly controlled dictatorship, no one in the Russian government had anything to do with Nemtsov’s murder?

We may never know the answer. Possible culprits range from Russia’s security apparatus to one of the extreme nationalist movements emboldened by Putin’s climate of fear and paranoia. In a cynical regime like Putin’s, the last thing we should expect is for the truth to emerge. Indeed, Kremlin apologists pointed to all the usual suspects in the killing’s aftermath — Chechen terrorists, the Ukrainian government, Russia’s democratic opposition itself, and even the United States. The Kremlin knows what to do at a moment like this — bury the truth deep beneath the Russian tundra, where no one will ever find it.

Nemtsov’s death, which prompted thousands of Russians to take to the streets in protest, illuminates anew the problem of a resurgent Russia under Putin, which is determined to repress dissent at home and to intimidate its neighbors through coercion. The United States and Europe need to shine a spotlight on repression in Russia. They also need to be stronger in opposing Putin’s aggression beyond his borders.

Putin is methodically intimidating neighboring states to create strategic depth in order to shield Russia from the sea of westward leaning democracies that emerged in Europe after the Cold War’s end. That is why Putin has dismembered Ukraine and annexed Crimea, in flagrant violation of the UN charter.

That is also why it is important for President Obama and European leaders to unite more effectively to contain Putin’s expansionist aims. But, as the New York Times’s Steve Erlanger wrote on Sunday, building a strong Western coalition against Putin is proving to be difficult. One reason is that our most powerful ally, Germany, is too conflicted about its break with Russia, and, in Erlanger’s words, too “ambivalent” about its own role in Europe to lead the Western response. The mantle of leadership thus falls to President Obama and the United States, as it has during every major European security crisis since the end of World War II.

This poses some difficult dilemmas for Obama. Until now, he has let German Chancellor Angela Merkel lead NATO’s response to Putin. But it was Merkel and French President Francois Hollande who agreed to the recent, deeply flawed Ukraine cease-fire that Putin and pro-Moscow separatists openly violated. Despite her many strengths and sterling character, Merkel appears unwilling to combine diplomacy with the tougher measures needed to contain Putin’s territorial ambitions.

Obama is the only Western leader who can play this role. He needs to convince Europe to lead a substantially stronger economic aid program for Ukraine’s beleaguered economy and to move toward stronger sanctions against Putin. In this respect, Washington has confidence in the new Ukrainian finance minister in Kiev, Natalie Jaresko, who is trying mightily to right the economy for the long term.

Obama can also increase pressure on Putin by sending defensive weapons to Kiev. And he should consider a new move on the chessboard — to station permanently a stronger NATO ground and air contingent in Estonia, Latvia, and Lithuania to shield those three allies from Putin. Former NATO Secretary General Anders Fogh Rasmussen, currently a fellow at Harvard, told me this is the most effective way to reaffirm NATO’s Article V security commitment to Baltic sovereignty.

In the wake of Nemtsov’s murder and Russia’s aggression in Ukraine, there is no other alternative for a stronger US-led coalition to contain the brooding, paranoid Putin — the greatest menace to a democratic Europe in this century.