What significant event is commonly referred to when talking about the September 11 attacks?

The significant event commonly referred to when discussing the September 11 attacks is indeed described as terrorists attacking the U.S. On September 11, 2001, a coordinated series of attacks were carried out by the terrorist group al-Qaeda, which involved hijacking commercial airplanes and using them as weapons against key targets in the United States. This attack led to the destruction of the World Trade Center in New York City and extensive damage at the Pentagon in Washington, D.C. The events of that day had profound implications for U.S. foreign and domestic policy, resulting in the initiation of the War on Terror and the establishment of various security measures aimed at preventing future attacks.

Other options listed, such as the U.S. Civil War, the Vietnam War, and World War II, do not pertain to the specific events of September 11, 2001, and therefore do not capture the essence of what this day represents in American history. These wars have their own significance and context but are unrelated to the terrorist attacks that took place on 9/11.