Today, there is no such cop. America is in retreat, gutting its military, swaddling its economy in projects that would have delighted Soviet central planners, and backing away from its role as leader of the free world. Putin is in the business of reassembling a Russian empire, and when the opportunity presents itself — why not?

President Obama’s statement that “there will be costs” is by now one more item in a list of ever less credible U.S. statements and warnings to the world’s opportunists. It sounds much of a piece with his response to North Korea that “rules must be binding. Violations must be punished. Words must mean something.” Or the statement to Iran, while “bearing witness,” that “the arc of the moral universe is long, but it bends toward justice.” Or the “red line” for Syria. Or “I don’t bluff” about Iran’s nuclear bomb project.

Nor does it help that Obama has yoked this latest warning to the “international community” — as in, “the United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine.” For a summary of what the international community amounts to, look to the United Nations, a feckless collective where freedom is profoundly out of style. In Libya, standing with the community — or leading from behind — sufficed to dispense with a beleaguered dictator whose people were already in open revolt. That’s a tactic unlikely to impress Putin’s Russia.

The stakes here are huge. It is not only Russia that is now exploring the limits of that question — why not? A militarizing and restive China is no doubt watching. The world is watching. If America does not lead, on terms that even Putin must take seriously, the next loaded question is, who will?