BAGHDAD — A startling spasm of violence shook more than a dozen Iraqi cities Monday, killing over 100 people in coordinated bombings and shootings and wounding twice as many in the country's deadliest day in more than two years.

The attacks came only days after al-Qaida announced it would attempt a comeback with a new offensive against Iraq's weakened government.

With the U.S. military gone and the government mired in infighting, the Iraqi wing of al-Qaida has vowed to retake areas it once controlled and push the nation back toward civil war. Though there was no immediate claim of responsibility for Monday's attacks, nearly all of them struck in the capital and in northern Iraqi cities where al-Qaida can most easily regain a foothold.