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2,977 people were killed when Al Qaeda hijackers seized four planes and flew them into the Twin Towers of the World Trade Centre in New York and the Pentagon in Washington.
The fourth plane was crashed into the ground when passengers and crew fought back.
It was the deadliest attack in history and set a chain of events that we are still living with.
25 years later, the four ex-presidents who dealt with the legacy of the attacks gathered as usual at ground zero in New York to remember the dead. But President Trump went to the Pentagon and made a speech in which he drew a line from 911 to his current war in Iran.
