WASHINGTON — President Obama’s first two defense secretaries publicly questioned the administration’s handling of the Syrian crisis on Tuesday night and expressed skepticism about whether Russia can broker a deal to remove Syria’s chemical weapons.

In a joint appearance in Dallas, both former Pentagon chiefs, Robert M. Gates and Leon E. Panetta, were critical of Mr. Obama for asking Congress to authorize the use of force against Syria in retaliation over its use of chemical weapons. But they disagreed on whether military action would be an effective response. Mr. Gates said Mr. Obama’s proposed military strike was a mistake, while Mr. Panetta said it was a mistake not to carry out an attack.

“My bottom line is that I believe that to blow a bunch of stuff up over a couple days, to underscore or validate a point or a principle, is not a strategy,” Mr. Gates said during a forum at Southern Methodist University. “If we launch a military attack, in the eyes of a lot of people we become the villain instead of Assad,” he added, referring to President Bashar al-Assad of Syria.