Final answer:
Terrorists first used planes as weapons on September 11, 2001, when Al-Qaeda hijacked four airliners and crashed them into the World Trade Center and the Pentagon, killing nearly 3,000 people and prompting the War on Terror.
Step-by-step explanation:
September 11, 2001: The Emergence of a New Terrorist Tactic
Terrorists using planes as weapons first appeared on the radar on the morning of September 11, 2001, when nineteen members of the extremist group Al-Qaeda executed coordinated attacks against the United States. Under the leadership of Osama bin Laden, these terrorists hijacked four commercial airliners. Two planes were flown into the Twin Towers of the World Trade Center in New York City, one into the Pentagon in Washington D.C., and the fourth, United Airlines Flight 93, crashed in Pennsylvania after passengers attempted to overcome the hijackers. The attacks on this day represent a new scale and method of terrorism, involving hijacking airliners and using them as guided missiles to cause mass casualties and destruction.
This violent tactic was unprecedented and chillingly effective, resulting in the deaths of nearly 3,000 people and the subsequent launch of the War on Terror, a global campaign led by the United States and allied nations to combat terrorism.