He saw action in the China-Burma-India Theater of World War II, first with Merrill's Marauders, getting wounded in combat, and then as a guerilla leader for the Office of Strategic Services.

He was an advocate of a strategy that emphasized the political nature of the conflict as much as the military aspect and was a proponent of the removal from power of South Vietnamese president Ngô Đình Diệm.

[4][9] After spending a year at Millard's Preparatory School in Washington, DC,[6] and another traveling around Europe, including a visit to Nazi Germany,[5] Hilsman attended the United States Military Academy and graduated in 1943[1] with a B.S.

[6] Following U.S. entry into World War II, Hilsman's father, a colonel, fought under General Douglas MacArthur during the Japanese invasion of the Philippines.

[4] After leaving West Point the younger Hilsman was immediately posted to the South-East Asian Theatre and joined the Merrill's Marauders long-range penetration jungle warfare unit, which fought the Japanese during the Burma Campaign.

[8] He participated in infantry operations during the battle for Myitkyina in May 1944 and suffered multiple stomach wounds from a Japanese machine gun while on a reconnaissance patrol.

[4] He then volunteered to be put in command of a guerrilla warfare battalion, organized and supplied by OSS Detachment 101, of some 300 local partisans, mercenaries, and irregulars of varying ethnicities, operating behind the lines of the Japanese in Burma.

[4][8] By early 1945, Hilsman was considered, as Detachment 101 commander William R. Peers later stated, to be one of a number of the guerillas' "good... junior officers, every one outstanding and experienced.

[8] In one particular engagement in May 1945, Hilsman led a mixed company of Kachins, Burmese, and Karens in staging successful raids in the area between Lawksawk and Taunggyi that culminated in a carefully-orchestrated ambush that caused a hundred casualties among the Japanese at no cost to the guerillas.

[11] Hilsman wanted to deploy his unit farther south into the Inle Lake area but was constrained by orders to help hold the road between Taunggyi and Kengtung.

[2] He worked on planning for the North Atlantic Treaty Organization and of Supreme Headquarters Allied Powers Europe with the Joint American Military Advance Group in London in 1950 to 1952 and as part of the International Policies Division of the United States European Command in Frankfurt, Germany, in 1952 to 1953.

[4] During staffing of the incoming Kennedy administration, the nominee for Under Secretary of State, Chester Bowles, aggressively sought academics and journalists who would be committed to the ideals of the New Frontier.

[13] In line with this, Hilsman was selected to be the Director of the Bureau of Intelligence and Research for the U.S. Department of State,[13] assuming the position in February 1961.

[4] His background in guerrilla warfare led him in 1961, together with Walt Rostow, to push for the American armed forces and the State Department to emphasize counterguerrilla training.

"[1] It stated that the war was primarily a political struggle and proposed policies that emphasized that the Vietnamese in rural areas were the key to victory.

[1] Out of the report came Kennedy's approval of American participation in the Strategic Hamlet Program, the relocation of rural peasants into villages consolidated and reshaped to create a defensible, networked perimeter, with the goal of removing population from contact and influence with the Viet Cong.

At the rate it is now going the war will last longer than we would like, cost more in terms of both lives and money than we anticipated...."[20] The report thus contributed to the escalation of American involvement in Vietnam and to growing doubts in U.S. government circles about the usefulness of the Diệm regime.

[19] In March 1963, the White House announced that Hilsman would become Assistant Secretary of State for Far Eastern Affairs, replacing Averell Harriman (who was promoted to an undersecretary position).

[22] On August 24, 1963, in the wake of raids against Buddhist pagodas across the country by Nhu's special forces, Hilsman, along with Forrestal and Harriman, drafted and sent Cable 243, an important message from the State Department to U.S.

[23] The events have also long been criticized as at best an example of a bizarrely poor decisionmaking process[22] and at worst a case in which a small group of secondary, anti-Diệm figures was able to circumvent normal procedures with a consequent harmful effect on the situation in Vietnam.

[12] Hilsman was one of the academics and intellectuals in the administration who were later grouped by the author David Halberstam in his book as The Best and the Brightest for the misguided foreign policy that they crafted and its disastrous consequences.

But Johnson sought a narrower range of opinion on foreign policy matters than Kennedy had and Hilsman, along with a number of other formerly influential State Department figures, was now not being listened to.

[27] At the same time, Hilsman disagreed with Johnson's approach to the Vietnam War, viewing the new president as primarily seeking a military solution there rather than a political one.

[6] The course he gave on foreign policy decision-making became known for the anecdotes he told about the famous figures in the Kennedy administration and for the political theory he introduced in explanation.

[16] Hilsman lived in Morningside Heights, Manhattan,[37] but he and his family also became longtime residents of the Hamburg Cove area of Lyme, Connecticut, for weekends and summers.

[29] The New York Times Book Review called it a "highly informative study of the internal and external forces that shaped much of American foreign policy" and said that "Hilsman makes many wise and perceptive comments on the politics of policy-making.

Hilsman continued to speak publicly, in print and on television, regarding what he thought should be done in Vietnam, such as in August 1964, when he warned against over-militarizing the conflict,[5] and in mid-1967, when he said the war was not politically "winnable" and that the U.S. should scale down its military involvement and stop the ongoing bombing campaign against the North.

[36] He predicted his chances of winning were directly linked to Democratic presidential nominee George McGovern's performance in the state against Richard Nixon, the incumbent whom Hilsman termed a threat to civil liberties.