BAGHDAD — Car bombs and gunmen struck new American allies, police, and civilians yesterday in northern Iraq, killing as many as 53 people. The spate of attacks came even as the American military released a captured diary and another document they say show Al Qaeda in Iraq cracking under a Sunni revolt against its brutal tactics. The violence coincided with a visit by Defense Secretary Gates to Baghdad, where he warned that hard choices face Iraq’s political leaders on how to stabilize the country despite promising new signs of progress toward reconciliation.

The deadliest bombing yesterday was near Balad, 50 miles north of Baghdad, against a checkpoint manned jointly by Iraqi police and members of an awakening group. Iraqi police said a suicide truck bomber targeted a checkpoint manned by American-allied fighters and Iraqi police at the entrance of a bridge in the district of Yathrib on the outskirts of Balad. Security forces opened fire on the driver, but he managed to detonate his payload, devastating a nearby car market and other stores.

It was one of the worst bombings this year amid a recent lull in violence and underscored American warnings that Al Qaeda in Iraq remains a serious threat despite military offensives that have severely curtailed its operations.

Meanwhile, a military jury yesterday convicted an Army sniper of murder and sentenced him to 10 years in prison for killing an Iraqi civilian who wandered into the hiding place where six soldiers were sleeping.

Sergeant Evan Vela, 24, was found guilty of murder without premeditation, of aiding and abetting in planting an AK–47 on the dead man’s body and of lying to military investigators about the shooting. He had faced a possible life sentence.