What type of attack was experienced by the United States on September 11, 2001?

On September 11, 2001, the United States experienced a terrorist attack orchestrated by the extremist group al-Qaeda. This event involved the hijacking of commercial airplanes, which were then used to target significant locations, including the World Trade Center in New York City and the Pentagon in Arlington, Virginia.

The nature of this attack was characterized by its use of civilian aircraft as weapons against unarmed civilians and symbolic government targets, marking it as a clear act of terrorism aimed at instilling fear, causing massive loss of life, and provoking a significant response from the U.S. government and military. This specific incident had profound and lasting impacts on both national security policy and international relations in the years that followed, leading to the War on Terror and heightened security measures domestically and abroad.

The other options such as military invasion, cyber attack, and nuclear attack do not accurately describe the nature of the 9/11 events, which were specifically designed to be acts of terror aimed at civilians and institutions rather than traditional warfare or attacks involving technology or nuclear weapons.