This domestic pressure was on display in Syria. Mr. Obama’s error was not that he backed away from military action and accepted Russia’s proposal to rid Syria of chemical weapons. The mistake was that he drew a red line that would have been more costly to back up than the United States was willing to tolerate. America lost credibility internationally for failing to make good on its threat.

Unfortunately, the Obama administration is repeating this mistake in Ukraine.

When Russia proceeded with the annexation of Crimea, the United States and Europe responded with punitive measures that had some economic impact. But they did not by any means “go to the hilt.” Instead, the Americans and Europeans drew an even deeper line in the sand, issuing empty threats of sweeping sanctions if Russia tried to grab more territory in Ukraine.

Such sharp rhetoric from the West could push Mr. Putin to be even more aggressive. That’s because he does not believe that the West would ever treat Russia like Iran and implement robust sanctions that would cut off vast areas of Russia’s economy from the West. As Mr. Putin recently explained, in a globalized world “it’s possible to damage each other — but this would be mutual damage.”

“Isolating Russia” as if it were Iran or North Korea isn’t a threat America can feasibly make good on. Just because Mr. Putin is acting like the leader of a rogue state, his country cannot be considered as such. Russia boasts the world’s eighth-largest economy. Given the exposure of American corporations to Russia, there would be serious pushback from the private sector if Mr. Obama tried to relegate Russia to rogue-state status. The Obama administration needs to preach what it will ultimately practice. Otherwise Washington’s credibility will erode further as it walks back its words.

A more hard-line response is not the answer. Mr. Obama was right to rule out the military option; diplomacy is America’s only viable path forward.

But Washington needs to anticipate a Russian response from a Russian perspective. In a major speech on Wednesday, Mr. Obama hinted that further sanctions would be implemented if Russia maintained its present course. That is a mistake. Russia will not back down, and such talk will only ratchet up tensions.

The Obama administration should focus on supporting Kiev rather than punishing Moscow. That means using its leverage with Europe to ensure that this support sticks, and that Ukraine’s new government does nothing to provoke an extreme response. This will require an acknowledgment of Russia’s core interests and America’s limitations — and an end to empty threats.