A small ceremony on Tuesday marked a big moment in Harvard history: the official opening of the first Reserve Officers’ Training Corps (ROTC) office on the Harvard campus in 40 years.

Harvard had banished any physical campus presence for ROTC programs in 1970 and 1971, at the height of protest over the Vietnam War. But in March the University signed an agreement with the Navy to bring an ROTC office back to campus, predicated on the expiration of the “Don’t Ask, Don’t Tell” law, which banned gays from the military. The law expired Sept. 20.

The office, two adjoining rooms in Hilles Hall on the Quadrangle, will serve as an advisory space for a Naval ROTC (NROTC) unit that is part of a consortium of six local colleges. Nine midshipmen from Harvard are enrolled in the unit, called the Old Ironsides Battalion. They will continue to take military science classes at the Massachusetts Institute of Technology, where Harvard ROTC Navy midshipmen have trained since 1976, alongside Army and Air Force cadets.

During a late-afternoon reception, a common room near the new second-floor office was crowded with men and women in uniform, who mingled with University officials. At one edge was a scrum of television cameras and reporters.

Naval officers and Harvard midshipmen listened as President Drew Faust delivered a brief keynote in a Hilles Hall common room. “The revival of the relationship between Harvard and the Naval ROTC,” she said, “marks an important new chapter in the long and storied history of military service by members of the Harvard community.”

“There’s no greater crucible of leadership than you can get in the military,” said retired Navy Capt. Paul E. Mawn, a 1963 graduate of Harvard College and a former ROTC midshipman. He is chairman of Advocates for Harvard ROTC, an organization with 2,600 members — three-quarters of them graduates of the College.

Mawn, who served in the Navy from 1963 to 1991, called the new ROTC office “a very important beachhead, but the mission’s not accomplished yet. There’s still a lot to be done.” Army and Air Force ROTC units should get a presence on campus as soon as possible, he said, and Harvard should alter its admissions policy by adding another category of diversity — veterans.

Harvard President Drew Faust delivered a brief keynote in the common room. “The revival of the relationship between Harvard and the Naval ROTC,” she said, “marks an important new chapter in the long and storied history of military service by members of the Harvard community.”

Faust, a historian of the Civil War, quoted Charles Russell Lowell, valedictorian of the Class of 1854 and a Union Army brigadier general who was mortally wounded in an 1864 battle. “The world,” said Lowell, “always advances by impossibilities achieved.”

Joining Harvard to ROTC once again represents a “society advanced,” said Faust, since it comes with the opening of the military to gay and lesbian citizens. “It not only affirms our shared interest in an inclusive society,” she said, “but also deepens the reservoirs of talent on which the military so vitally depends.”

Harvard College Dean Evelynn M. Hammonds, who introduced Faust, remarked on the University’s “long commitment to military service as among the highest forms of public service.”

She also praised Faust for her “outspoken and courageous commitment to ROTC — and to military service as an expression of public service and diversity.”

Also speaking at the ceremony was U.S. Navy Capt. Curtis R. Stevens, commanding officer of the BU-MIT Naval ROTC Consortium — and now director of Naval ROTC at Harvard University. “Today Harvard makes a tangible statement,” he said, “in that the education of future military leaders is an important function of a great university in a free and open society.”

Stevens, a nuclear engineer who started his career in the submarine service, touched on Harvard’s historical ties to the military — remarking that George Washington took command of the Continental Army while headquartered in Harvard Yard; that in 1926 the Harvard Naval ROTC unit was among the first six nationwide; and that many notable Harvard alumni had ties to the Navy, including John F. Kennedy ’40, and Theodore Roosevelt, Class of 1880 — a Medal of Honor recipient who is considered “the father of the modern American Navy,” said Stevens.

After making their remarks, Faust, Hammonds, and Stevens joined to cut a crimson-white-and-blue ceremonial ribbon in a hallway leading to the new office. Faust presented one segment of ribbon to Nicholas A. Christakis, a Harvard Medical School professor who co-chaired the Harvard-ROTC implementation committee named in March.

Fellow co-chair Kevin Kit Parker, an Army major, could not attend. He is now on his fourth tour of duty in Afghanistan. Parker is the Tarr Family Professor of Bioengineering and Applied Physics at the Harvard School of Engineering and Applied Sciences.

Watching the ceremony at Hilles, dressed in crisp whites, were all nine Navy ROTC midshipmen enrolled at Harvard College. If they had one message it was to thank all the ROTC cadets and midshipmen before them, who laid the groundwork for bringing a formal ROTC presence back to Harvard.

“I’m glad it’s back,” said Sebastian Raul Saldivar ’15, of Dallas, Texas — one of two freshmen in the Naval ROTC program. “It gives me faith in the University.”