President Donald Trump gave a rousing speech at the Pentagon to honor 9/11 victims … saying America remains committed to fighting terrorism in their memory.

POTUS took the stage Friday against a backdrop of American flags … promising to never forget all the people killed on that tragic day a quarter-century ago.

Trump declared … “That’s why we fight today. We don’t have a choice. There can only be victory. We fight hard; we fight to win.”

The Prez — who was living and working in New York City on 9/11 — says the day started out so beautiful … though it will always be a day that lives in infamy.

While the day is mostly about looking back, Trump says we will protect this country for future generations.

Trump’s Pentagon speech was just one of many remembrances for the 25th anniversary … he originally planned to go to Ground Zero, but pivoted to the Pentagon after being denied the chance to give a speech at Ground Zero, which is a long-held tradition.

Two planes flew into the Twin Towers in NYC on Sept. 11, 2001 … while a third plane crashed into the side of the Pentagon. More than 2,700 people were killed in New York while 184 were killed in D.C.

A fourth plane was headed toward Washington, D.C. … and investigators have concluded it may have crashed into either the U.S. Capitol Building or the White House if passengers hadn’t fought for control of the plane before crashing it into a Pennsylvania field.