WASHINGTON — The Iraqi Army's recent failure to defeat insurgents in Basra should serve as a lesson about what may happen if U.S. forces withdraw from Iraq, the commander of U.S. forces told Rep. John M. McHugh on Wednesday.

In response to questions from Mr. McHugh, R-Pierrepont Manor, at a House Armed Services Committee hearing, Gen. David H. Petraeus said the struggle in Basra illustrates why the United States should base troop drawdowns on "conditions on the ground," not timelines dictated by Congress.

"There have been gains — fragile gains," Gen. Petraeus said. But the situation in Basra showed how criminal elements were able to infiltrate port operations, for instance, and position themselves to defend against an offensive by Iraqi forces.

In the end, Iraqi troops were outmatched by Shiite militias as they tried to rid the port city of the insurgents' influence. Eventually, U.S. forces were drawn into the battle, which ended only after Muqtada al-Sadr, the radical cleric, ordered his fighters to stop.

Other violence has erupted since, including skirmishes in the Sadr City section of Baghdad.

"I guess the big debate is what happens with a precipitous withdrawal," Mr. McHugh said. He asked about the conditions that allowed the conflict to flare in Basra, and what that means if more U.S. forces leave the country — a situation that another lawmaker, Rep. Roscoe G. Bartlett, R-Md., later likened to unplugging a hole in a dike.

"There are lessons to be learned," Gen. Petraeus said. But it is important to remember, he said, that the Iraqi government, led by Prime Minister Nouri al-Maliki, started the fight in Basra with its own offensive.

Mr. McHugh has consistently supported the operation in Iraq and opposed troop withdrawal deadlines from Congress. He said Wednesday that if the Iraqi government did more, and other countries in that region did more, to promote peace, that he and other lawmakers would vote to bring U.S. troops home.

But the U.S. ambassador to Iraq, Ryan Crocker, has said that withdrawing U.S. forces would be "devastating" for Iraq and that Osama bin Laden has called the country a perfect base for terrorist operations, Mr. McHugh said.

Mr. Crocker, testifying with Gen. Petraeus, said al-Qaida has come close to setting roots in Iraq.

"If they are to succeed, it's certainly my judgment that the threat to the United States would rise considerably," Mr. Crocker said.