Twenty years ago, terrorists in hijacked airliners changed America—and the military—forever.

On the morning of Sept. 11, 2001, two hijacked planes crashed into the World Trade Center in New York City. Shortly afterward, at 9:37 a.m., American Airlines Flight 77 crashed into the west side of the Pentagon, shearing through three of the building’s five rings.

Finally, at 10:03 a.m., United Airlines Flight 93 crashed in Shanksville, Pennsylvania, after the passengers, upon learning about the attacks in New York and on the Pentagon, tried to retake the plane.

Some 2,750 people were killed in New York, 184 at the Pentagon and 40 in Shanksville.

As the nation pauses to mark the 20th anniversary of the attacks, retired Gen. Carter Ham, president and CEO of the Association of the U.S. Army, said he vividly recalls the events of that tragic Tuesday morning.

“Little did I understand how that morning would shape the remainder of my time in uniform and affect all of our lives in ways we simply couldn’t imagine at the time,” said Ham, who was then a newly assigned colonel on the U.S. Central Command staff.

Initial reports about a plane striking one of the World Trade Center towers led many to believe it was a terrible aviation mishap with no real military relevance. That changed when the second plane hit, Ham said.

“We at CENTCOM, as I am sure was the case in military headquarters elsewhere, intuitively understood that this was no mishap,” he said. “This was an attack. Planned and conducted by enemies of America.”

While the intelligence and evidence would come soon enough, no one at the Central Command headquarters in Tampa, Florida, had any doubt as to who was behind the attacks, Ham said.

“Rage was quickly replaced by an intensity of focus and purpose that I have rarely witnessed,” he said. “We were at war.”

Now, 20 years later, it can sometimes seem so challenging to remember how the country came together, putting aside disagreements and uniting for a common purpose, he said.

“We saw incredible courage amidst the carnage in New York, at the Pentagon and in the skies over Shanksville, and we resolved that those taken from us would not have been lost in vain,” Ham said. “In the intervening 20 years, the bravery and selflessness of first responders at home were matched on the battlefields by a generation of women and men who answered our nation’s call to duty.”

He added, “Far from home, and in more recent years, too often far from our nation’s thoughts, soldiers, sailors, airmen and Marines fought just as courageously as their forefathers. They, too, made awful sacrifices. We all owe them and their families a debt that can never truly be repaid.”

Today, some may ask if it was all worth it, Ham said. “My reply is an unequivocal ‘Yes.’ America’s armed forces did their duty,” he said. “They fought with honor and courage, adding to the histories of our military.”

The troops served selflessly. “We should remember that every service member who served over the past 20 years did so voluntarily,” Ham said. “Members of the National Guard and reserve lived up to the finest traditions of the citizen-soldier, a uniquely American characteristic and one we should never take for granted.”

Their families sacrificed, too, Ham said, including “some suffering the most unimaginable loss that comes with an unwelcome knock on the door. The troops, as they always do, served with distinction, bringing hope to the hopeless and striking fear into the hearts of America’s enemies. We owe those who served, and especially those we lost, our unending support and admiration.”

Twenty years can seem like a long time, Ham said. “Today, women and men serve in uniform who have no personal recollection of Sept. 11, 2001. Some weren’t yet born,” he said. “Yet still they have chosen to serve. I guess that’s why, on this 20th anniversary of such a terrible day, I still hold on to all that I treasure about America. Young people still, every day, step forward, raise their right hands to serve us, to defend us from those who wish us harm. That, at least to me, is the enduring legacy of 9/11—service to the nation, rising above one’s personal concerns, even personal safety and well-being, to answer the call to duty.”

Ham said he is proud to have served. “I am so incredibly proud to have served alongside those who have made a difference over the past 20 years and just as proud to see such amazingly talented and dedicated women and men continue to step forward as they say to our nation, ‘Send Me.’ ”