– A "dissent memo" signed by dozens of State Department officials urging U.S. airstrikes on Syrian President Bashar Assad's forces is a message to Hillary Clinton — and a warning to Assad and Russia — that career diplomats want President Obama's successor to take a harder line.

The memo called for limited U.S. airstrikes on Assad's forces in response to the collapse of a tentative cease-fire in Syria's five-year civil war. Assad said this month that he would take back "every inch" of Syria from opposition forces. The 51 officials who signed the document argued that U.S. military action would force Assad and his ally, Russia, to make concessions.

Clinton, the former secretary of state, already has signaled a preference for a more aggressive course in Syria than Obama, including enforcement of a no-fly zone to protect Syrian civilians. Presumptive GOP nominee Donald Trump has said that he would order more military action against the Islamic State in Iraq and the Levant but not against Assad.

White House spokeswoman Jen Friedman said the administration is always open to new ideas. Obama, she said, "has always been clear that he doesn't see a military solution to the crisis in Syria and that remains the case."

Robert Ford, who resigned as Obama's ambassador to Syria in 2014, said that in three decades at the State Department, he never saw a dissent memo signed by more than three or four officials.

"To get 51 signatures on a dissent channel message is remarkable and suggests a very broad consensus … that the current policy is failing," said Ford, now a senior fellow at the Middle East Institute.