VLADIMIR PUTIN’S assault on Ukraine has been relentless and increasingly reckless: Forces working with Russian personnel in eastern Ukraine are torturing and murdering opponents and holding international observers hostage. In contrast, President Obama’s response has been slow and excruciatingly measured. New U.S. sanctions announced Monday fall well short of the steps that senior officials threatened when the Russian offensive in eastern Ukraine began three weeks ago.

No wonder that, even as he announced them, Mr. Obama expressed skepticism that they would work. “We don’t expect there to be an immediate change in Russia’s policy,” a top aide told reporters. This official acknowledged that the United States could take steps that would impose “severe damage on the Russian economy” but was holding them back. The obvious question is: Why would the United States not aim to bring about an immediate change in Russian behavior that includes sponsorship of murder, torture and hostage-taking?

Mr. Obama said the sanctions, aimed at business cronies of Mr. Putin and their firms, are “calibrated” to “change his calculus.” As in the failed attempt to change the calculations of Syrian President Bashar al-Assad, the White House is assuming that a ruler engaged in wanton aggression can be gently steered to an off-ramp with half measures. The strategy was worth trying after the Ukraine crisis began in late February, but the Russian president, like Mr. Assad, has made a mockery of the administration’s diplomacy, blatantly ignoring the agreement accepted by his foreign minister in Geneva 11 days ago.

U.S. officials say that “sectoral” sanctions against Russian banks and the energy and mining industries are being held in reserve as a deterrent against a Russian invasion of Ukraine. But that seems to imply a writing-off of Moscow’s annexation of Crimea and its systematic and increasingly brutal effort to create chaos in eastern provinces. And hasn’t Russia already invaded Ukraine? Kiev’s intelligence service says at least 30 officers of the Russian military intelligence service have been directing the assaults on local governments; a White House statement Monday said, “Russia’s involvement in the recent violence in eastern Ukraine is indisputable.”

A better explanation was hinted at by a senior official who said the administration did not want to act without the European Union, which announced its own minimalist sanctions expansion Monday. The official also said that the administration needed to consider “the effect on the global economy.” That suggests the U.S. sanctions policy is “calibrated” less toward rescuing Ukraine than toward avoiding steps that would ruffle feathers in Brussels or set back U.S. economic growth in an election year.

Those are understandable motives, but they ought to be trumped by the imperative of standing unambiguously against the first forcible change of borders in Europe since World War II. By choosing not to use the economic weapons at his disposal and broadcasting that restraint to the world, Mr. Obama is telling Mr. Putin as well as other potential aggressors that they continue to have little to fear from the United States.