Tuesday was a special day for history in San Francisco as the Postal Service showed off its new stamp commemorating the centennial of World War I.

A gathering of old veterans and young men and women in uniform gathered to honor the memory of those who served in what they called “The Great War.”

“We are here to honor a generation that can no longer speak for itself,” said Col. Fred Rutledge, a historian who is director of museum operations for the California Military Department. “There are no more World War I veterans left, but they are a part of who we are and how we got here.”

Rutledge, who has more than 27 years of military service, wore the brown Army uniform of a century ago, complete with a leather belt and riding boots.

He paused for pictures with a color guard of ROTC cadets from the University of San Francisco, all of them wearing modern camouflage uniforms.

Bill Wang, a college freshman, said the cadets were there to pay respects to those who fought in the war. “We’re here to honor them and their service,” said Wang, who has not served in the military but said he is thinking about it when he finishes college.

However, the main emphasis at Tuesday’s ceremony was not about the future, but the past. “I think World War I was the most significant event of the 20th century,” said J. Michael Myatt, a retired Marine Corps major general who was one of the speakers at the short ceremony.

The armistice that ended the first world war 100 years ago next month was only a pause, Myatt said, to be followed by a second and even more terrible world war. “And we are still fighting it,” he said.

Myatt praised the unsung contribution of the Postal Service in wartime. He is a veteran of the Vietnam and Persian Gulf wars, both fought before the time of instant communications on the internet. These were wars when mail from home was really important, he said.

“I remember my first tour in Vietnam,” he said. “I spent 13 months there and never talked to my wife once. It was all by mail. When you got a letter from home, you didn’t read it once. You read it over and over. You read it 10 times. It was a great morale booster.”

In the internet age, mail from home is much less important. “I bet young people don’t even know what postage stamps are,” said Robert White, an Army veteran who is working on San Francisco’s Fleet Week.

Stamp collectors surely do. Many were on hand to buy the World War I stamps at 50 cents each. They also got special cachet envelopes with commemorative World War I drawings from the Army, Navy and Marines, including the women in wartime service, local heroes and the 369th Infantry regiment made up of African American soldiers known as the “Harlem Hellfighters.”

Each of these came with a special cancellation “War Memorial Veterans Building station, San Francisco CA, 94102.” The postmark made the envelopes one of a kind and valuable to collectors, said Ed Flowers, a retired Marine Corps sergeant and an avid stamp collector.

Postal Service spokesman Augustine Ruiz got in the last word for military mail. He met his wife on a blind date in Illinois while he was on leave from the Navy. They clicked, but he had to go back to sea aboard a submarine. They agreed to write.

The submarine rarely saw port, so mail call didn’t come often and her letters came in a bunch, dozens at a time. After months and months of writing back and forth, she agreed to marry him, thanks to the post office. “That was the greatest pitch letter I ever wrote,” he said. They have been married for 50 years.

Carl Nolte is a San Francisco Chronicle staff writer. Email: cnolte@sfchronicle.com Twitter: @carlnoltesf