President Obama on Thursday called on Syrian President Bashar al-Assad to resign due to his continued brutality against his own restive population.

Mr. Assad’s call for dialogue rings hollow at a time when he is unleashing tanks and attack helicopters against peaceful protesters in Hama, Deir al Zour, and other Syrian cities, said Mr. Obama.

“We have consistently said that President Assad must lead a democratic transition or get out of the way. He has not led. For the sake of the Syrian people, the time has come for President Assad to step aside,” said Obama in a statement.

The move was the toughest action yet taken by the administration against the increasingly embattled Assad regime. It was accompanied by new sanctions that freeze all Syrian government assets within the United States and prohibit US firms from dealing in any way in Syrian oil.

In addition, it was part of a coordinated verbal and legal multinational assault, as the leaders of Britain, France, Germany, and the European Union issued similar statements that Assad must go now.

But Obama’s Thursday action was also a long time coming. Assad’s apparent intent to hold on to power at any cost has been on display for weeks, as Syrian security forces have increased the violence of their crackdown.

The Obama administration called for the resignation of Egyptian President Hosni Mubarak fairly soon after protests arose in Egypt. Why did it take relatively longer in the case of Syria?

For one thing, the US wanted to make sure it was acting in coordination with allies, said a senior administration official. A call to Assad to resign delivered by many capitals at once has far more effect than one emanating from a single nation, even if that nation is the US.

“We wanted to make sure we weren’t just issuing statements [on our own], but were doing so in an internationally coordinated way,” said the official, speaking on background in a conference call with reporters.

In addition, the US slapped sanctions on Syria at the very beginning of its crackdown, said the official. In that sense the US has been tougher on Assad than it ever was on Mr. Mubarak, a longtime American ally.

“In Syria, unlike in Egypt, we pursued punitive measures from the beginning,” said the US official.

When the Syrian regime redoubled its efforts to suppress demonstrations with force at the beginning of Ramadan, it was clear that the time to take the final step and call for his resignation was getting close, said the official. At that point the administration realized that there was no chance of Assad recovering enough legitimacy to lead a Syria at peace.

“Increasingly we felt the need to coordinate a stronger response given the continued violence against the Syrian people,” said a second senior administration official.

Will Assad now actually resign as a result of international pressure? That seems unlikely. The example of Mubarak, who left voluntarily but now faces prosecution in Egypt, cannot be comforting to Assad.

At this point, Assad is in a “dictator’s dilemma,” according to Andrew Tabler, a fellow at the Washington Institute for Near Eastern Policy. He has unleashed so much violence that it would be extremely difficult for him to back down.

That said, concerted multilateral pressure on Syria has worked in the past, notes Mr. Tabler in an online interview posted on the Council on Foreign Relations website. That is what drove Syria out of Lebanon in April 2005, after the assassination of Lebanese Prime Minister Rafik Hariri.

“The second thing that is effective with the Syrians are sanctions, which have affected their economy greatly,” said Tabler.

Writing Thursday in his popular Foreign Policy magazine blog, Daniel Drezner, professor of international relations at the Fletcher School of Law and Diplomacy at Tufts University, questioned whether today’s US moves will accomplish much of substance.

That does not mean that publicly calling on Assad to resign is a bad thing, said Mr. Drezner. Such rhetoric may serve as the policymaker’s equivalent of blowing off steam, releasing pressure that otherwise might drive the US toward more risky policy options.

“When the rest of the policy quiver has been exhausted, sure, why not call for Assad to leave? As a general rule, all else equal, I see no reason why the US government should not express its actual preferences rather than hide behind diplomatese,” said Drezner.