People march during a march in memory of Russian opposition leader and former Deputy Prime Minister Boris Nemtsov on March 01, 2015 in central Moscow, Russia. (Alexander Aksakov/Getty Images)

RUSSIANS KNOW all too well the possible consequences of joining protest rallies in Moscow. Police regularly attack and beat peaceful demonstrators, and some have been sentenced to long prison terms. Boris Nemtsov , one of the principal organizers of those rallies, was brazenly gunned down on a heavily secured bridge near the Kremlin late Friday. So it would have been understandable if the crowd for a march held in his honor Sunday had been sparse.

Instead, tens of thousands showed up, defying icy rain as well as the fear that pervades Vladi­mir Putin’s Moscow. The mood was somber, even though security forces — no doubt aware that the world was watching — held back. In contrast to the big rallies that Mr. Nemtsov helped to lead three and four years ago, this one inspired little hope of a larger revolt against the Putin regime in the near future. It showed, however, that many Russians, like Mr. Nemtsov himself, did not give up the cause of democracy and human rights. They’re still willing to oppose the lies, corruption, imperialism and repression of Mr. Putin — and they deserve the support of the democratic West.

Sadly, many Western leaders remain in a state of denial about Mr. Putin or still hesitate to take measures that might stop his aggression. The Obama administration has delayed for weeks any decision on imposing new economic sanctions or supplying defensive weapons to the Ukrainian government, despite the failure of Russian forces in eastern Ukraine to respect a Feb. 12 cease-fire agreement. On Feb. 21, Secretary of State John F. Kerry said he was “confident some additional steps will be taken in response to the breaches of the cease-fire.” On Feb. 25, he said, “we are getting to that critical decision time.”

Nothing happened.

The administration, which often boasts of acting in lockstep with allies on Ukraine, has not even matched the modest additional sanctions adopted by the European Union on Feb. 17. Though both his secretary of defense and his director of national intelligence have publicly indicated their support for delivering weapons to Ukraine, President Obama has remained passive. On Monday, after a lengthy meeting with Russian foreign minister Sergei Lavrov, Mr. Kerry announced that Russia was getting yet another chance. There would be consequences, he said, if the violations of the cease-fire “continue.”

Many Russians, including Mr. Nemtsov, have tried to point out that such irresolution encourages Mr. Putin to escalate his campaign to forcibly overturn the post-Cold-War order in Europe. They say he can be stopped only by steps that decisively raise the cost of his military aggression and cripple the financial system that sustains his regime. Mr. Nemtsov was seeking to help in his own way, by preparing a report documenting the participation of Russian troops in the Ukraine war — something that Mr. Putin denies in part because his public opposes it.

It’s not known who murdered Mr. Nemtsov, and it probably won’t be as long as Mr. Putin remains in power. However, the Russians who courageously turned out on Sunday placed responsibility where it must lie: They marched straight to the Kremlin. If only Western leaders would show some of the same determination, Mr. Putin might still be checked.