The Revolutionary War closed the college from December 1776 until September 1782. When British and Hessian troops under Sir Peter Parker landed in Newport on December 7, 1776, President Manning wrote,

“This brought their Camp in plain View from the College with thenaked Eye; upon which the Country flew to Arms & marched for Providence, there, unprovided with Barracks they marched into the College & dispossessed the Students, about 40 in Number."

Four of the seven graduates from Brown’s inaugural class, the Class of 1769, served during the Revolution. William Rogers, the first student, was the chaplain of Miles's Pennsylvania Rifle Regiment, and later a brigade chaplain in the Continental Army. Richard Stites, the second student, was a captain in Heard's brigade, General Nathanael Greene's division of the Continental Line. He died of wounds received during the battle of Long Island on August 27, 1776, the first Brown graduate to die in military service.

When the Civil War began Brunonians, once again, quickly found a role to play. Excerpts from the diary of Henry S. Burrage, Class of 1861, recorded at Brown occasioned by the attack on Fort Sumter on April 12, 1861:

“Monday, April 15th. The seniors had a meeting after the Dr.’s recitation, and appointed a committee to obtain permission to raise the stars and stripes over the college building. The Dr. assented and as soon as the flag staff can be erected and the flag secured the national ensign will wave over halls which once served as barracks for the heroes of the Revolution. . .”

On May 11 a meeting of undergraduates voted to form a military company and June 8 were able to "parade in their new unifonns, dark blue shirts, light blue Zouave pants, red cap." The Cadets even saw active service in the early summer of 1863 when a Confederate privateer threatened to sail up to Providence. The cadets were eager to go, especially as they would miss their examinations. For fourteen days they built fortifications and did guard duty. When the danger passed, each received his $5.63, private's pay, and they all presented "resolutions" to the faculty that they be excused from the examinations they had missed. The result was that they were required to take the examinations at the beginning of the next term. They did, however, enjoy a little song, of which one verse was:

We won't pass examinations,

Oh, no! not we, not we.

We all went down to West Passage

for five and sixty-three.

Elisha Benjamin Andrews (1844-1917), the 8th President of Brown University,  was a Civil War Veteran. Andrews was born in Hinsdale, New Hampshire, on January 10, 1844. He attended the Connecticut Literary Institute in Suffield, but left to enlist in the First Connecticut Heavy Artillery when the Civil War broke out. He rose to the rank of second lieutenant, and was severely wounded in a battle on the James River on August 24, 1864. He lost the sight of his left eye, which was replaced by a glass eye in 1884. He was mustered out of the army in October 1864.

World War II brought year-round operation to Brown and turned the campus into a military training ground. President Wriston's message to the students on campus was:

"The armed forces need a steady flow of educated personnel. To this end, the Secretaries of War and of the Navy have issued a joint statement which first emphasizes that the country can no longer afford to have young men proceed with their education at the regular tempo, and second, urges those who are willing to put forth an intensive effort to continue in college."

Cooperating in the war effort, Brown sought and secured a Naval Reserve Officers Training Corps, and Brown's first V-7 training course had more students than any other New England college. A Naval Reserve Officers Training Corps unit was established at Brown in the fall of 1940 with 110 members. Capt. Chester H. J. Keppler became executive officer of the ROTC and chairman of the Department of Naval Science and Tactics with headquarters in the basement of Maxcy Hall. Students who were admitted to the Corps under peace-time conditions before March 1943 took a four-year course in naval science and tactics which, together with a Summer cruise, entitled them to a degree and a commission as Ensign in the Volunteer Naval Reserve or Second Lieutenant in the Marine Corps Reserve. As the third semester of academic year 1942-43 began in February, 1943, nearly 300 students who were members of the Army Enlisted Reserve Corps left for active duty. The Naval ROTC unit had 237 men expected to be commissioned in July.

When the news of Japan's surrender came, the ringing of the University Hall bell brought students, civilian and Military, out of their dormitories to unite with an impromptu band in a parade to the Pembroke campus, where the women joined the procession. They then went down Thayer Street and stopped at the flagpole for the singing of the "Star-Spangled Banner." Classes were suspended and a convocation called in Sayles Hall, at which Bruce Bigelow delivered a moving speech citing Brown's contribution and honoring those who had died. The former students of the University who lost their lives in the war numbered 177, ranging from the Class of 1907 to the Class of 1947 and including four graduate students and one woman.

On July 1, 1951 Brown University, along with 61 other institutions, was notified that it would be among the first campuses to host Air Force ROTC units. President Wriston received the Brown unit hospitably, "Under the current circumstances, it is a public service the University can render, and I am delighted that the Air Force has given Brown this opportunity.”

The ROTC units were highly popular throughout the 50s and 60s, until the ongoing Vietnam conflict eroded support and enthusiasm for military service among both students and faculty. A long-term debate on the future of ROTC began in 1967, when 35 members of the Brown Committee to Abolish ROTC picketed the annual ROTC spring review at Meehan Auditorium.

In November 1967, the Cammarian Club (the student government organization) passed a resolution stating that the ROTC program was not compatible with the academic integrity of the University. The Brown Chapter of the American Association of University Professors concurred.

The annual review was picketed again in 1968. In March 1969 the faculty rejected by a vote of 115 to 55 a resolution calling for the end of all military education, but recommended that the ROTC units should not have departmental status, that instruction by these units should not earn academic credit, that their officers should not have faculty status, and that the programs should be considered extracurricular activities. The University entered into new agreements for phaseout periods for the two ROTC programs, which applied only to students enrolled as of July 1, 1969. Negotiations were reopened in 1971, and, since the stipulations of the March 1969 resolution were not being met, the faculty affirmed its action and ended the presence of the ROTC at Brown when existing contracts expired, in 1971 for the Air Force unit and in 1972 for the Naval unit. Brown, however, has maintained its affiliation with the Patriot Battalion at Providence College,  a U.S. Army R.O.T.C program, continuously since 1951.