Speaking to 4,000 members at the National Guard Association convention, Romney said the National Guard in its long history has "never faltered, never wavered," whether called up to help residents at home recover from natural disasters or to fight an enemy in distant lands.
"Our world is a dangerous place," he said. "And the attack on Sept. 11, 2001, reminds us that the mission of the Guard is ever more critical and ever more deserving of our support and honor."
Some 3,000 died when two hijacked commercial jetliners crashed into the Twin Towers of the World Trade Center in New York City; another crashed into the Pentagon; and a third crashed in a Pennsylvania field.
Romney said he was in Washington, D.C., that day as the tragic events unfolded, and drove near the Pentagon.
"Cars had stopped where they were, and people had gotten out, watching in horror," he said. "I could smell burning fuel and concrete and metal. It was the smell of war, something I never imagined I would smell in America."
Both Romney and President Barack Obama suspended the biting tone of campaigning Tuesday, in deference to the solemnity of the day.
"With less than two months to go before Election Day, I would normally speak to a gathering like this about the differences between my and my opponent's plans for our military and our national security," Romney said.
"There is a time and a place for that, but this day is not it."
Romney, however, used the occasion to pledge his support for a strong military, Veterans Affairs system and preservation of the Department of Defense budget.
"We can all agree that our men and women in the field deserve a clear mission, that they deserve the resources and resolute leadership they need to complete that mission and that they deserve a country that will provide for their needs when they come home," he said.