In Milwaukee, nine members of the Milwaukee Police Department are killed by a bomb, the most deaths in a single event in U.S. police history until the September 11 attacks in 2001.

Milwaukee is the most populous city in the U.S. state of Wisconsin. Located on the western shore of Lake Michigan, it is the 31st-most populous city in the United States and fifth-most populous city in the Midwest with a population of 577,222 at the 2020 census, while the Milwaukee metropolitan area with over 1.57 million residents is the 40th-largest metropolitan area in the nation. It is the county seat of Milwaukee County.

The Milwaukee Police Department (MPD) is the police department organized under the city of Milwaukee, Wisconsin. The department has a contingent of about 1,800 sworn officers when at full strength and is divided into seven districts. Jeffrey B. Norman is the current chief of police, serving since December 2020.

The Milwaukee Police Department bombing was a November 24, 1917, bomb attack that killed nine members of local law enforcement and a civilian in Milwaukee, Wisconsin. The perpetrators were never caught but are suspected to be an anarchist terrorist cell operating in the United States in the early 20th century. The target was initially an evangelical church in the Third Ward and only killed the police officers when the bomb was taken to the police station by a concerned civilian. The bombing remained the most fatal single event in national law enforcement history for over 80 years until the September 11 attacks.

The September 11 attacks, also known as 9/11, were four coordinated Islamist terrorist suicide attacks by al-Qaeda against the United States in 2001. Nineteen terrorists hijacked four commercial airliners, crashing the first two into the Twin Towers of the World Trade Center in New York City and the third into the Pentagon in Arlington County, Virginia. The fourth plane crashed in a rural Pennsylvania field during a passenger revolt. The attacks killed 2,977 people, making it the deadliest terrorist attack in history. In response to the attacks, the United States waged the global war on terror over multiple decades to eliminate hostile groups deemed terrorist organizations, as well as the governments purported to support them.