Answer: terrorist attacks.
Explanation: The terror attacks on September 11, 2001, were the deadliest attacks on US soil since the Pearl Harbor bombing that launched the US into World War II. The plane hijackings that struck the World Trade Center, the Pentagon, and a Pennsylvania field killed 2,977 people. The attacks started at 8:46 a.m., with American Flight 11 hitting the World Trade Center's North Tower. The attacks dramatically changed the US, and started the "War on Terror," which targeted the Al-Qaeda extremist group and its leader, Osama bin Laden. I hope this helps you:)