The "Winter Soldier Investigation" was a media event sponsored by the Vietnam Veterans Against the War (VVAW) from January 31, 1971, to February 2, 1971. It was intended to publicize war crimes and atrocities by the United States Armed Forces and their allies in the Vietnam War. The VVAW challenged the morality and conduct of the war by showing the direct relationship between military policies and war crimes in Vietnam. The three-day gathering of 109 veterans and 16 civilians took place in Detroit, Michigan. Discharged servicemen from each branch of the armed forces, as well as civilian contractors, medical personnel and academics, all gave testimony about war crimes they had committed or witnessed during the years 1963–1970.

With the exception of Pacifica Radio, the event was not covered extensively outside Detroit. However, several journalists and a film crew recorded the event, and a documentary film titled Winter Soldier was released in 1972. A complete transcript was later entered into the Congressional Record by Senator Mark Hatfield, and discussed in the Fulbright Hearings in April and May 1971, convened by Senator J. William Fulbright, chair of the United States Senate Committee on Foreign Relations.

Prompted by numerous investigations into war crimes such as the Russell Tribunal, National Veterans Inquiry and Citizens Commission of Inquiry (CCI), the Vietnam Veterans Against the War wanted to have a large scale public hearing. With the courts martial for the My Lai Massacre making front-page news, and the recent disclosure by members of the Central Intelligence Agency's Phoenix Program of its record of human rights violations in Vietnam, the VVAW was determined to expose a broad pattern of war crimes in Vietnam. The Winter Soldier Investigation (WSI) was intended to prove that massacres like the My Lai were not isolated and rare occurrences, but were instead the frequent and predictable result of official American war policy.

The groundwork for what would become the Winter Soldier Investigation was laid by Jeremy Rifkin, Tod Ensign, Michael Uhl and Bob Johnson of CCI. In search of first-hand information on war crimes, they contacted the Vietnam Veterans Against the War and gained the support of VVAW co-founder Jan Crumb. During the summer of 1970, the CCI were approached by Al Hubbard who had become a full-time organizer with VVAW. Hubbard suggested that CCI combine their efforts with Jane Fonda, Rev. Dick Fernandez of Clergy and Laymen Concerned about Vietnam (CALCAV), Mark Lane, and Donald Duncan (who had previously testified at the Russell Tribunal in Denmark). An initial steering committee was formed, consisting of Duncan, Ensign, Fonda, Lane, Hubbard, Rifkin and Fernandez, and continued to organize the WSI through September, 1970.

Among the growing collective of organizers, differences of opinion and direction arose concerning the planned public event. VVAW leaders felt it should be an all-veteran event, to maintain its credibility. Less than three months into planning for the Winter Soldier Investigation, most of the Vietnam veteran organizers and Jeremy Rifkin had become adamant that WSI disassociate itself from Mark Lane. CCI staffers criticized Lane as being arrogant and sensationalistic, and said the book he was writing had "shoddy reporting in it." The CCI leaders refused to work with Lane further and gave the VVAW leaders a "Lane or us" ultimatum. VVAW did not want to lose the monetary support of Lane and Fonda, so the CCI split from the project. The following month, after caustic reviews of Lane's book by authors and a Vietnam expert, VVAW would also distance itself from Lane just one month before the event. Lane had recently published a book, Conversations with Americans, in which Lane relied on unverified interviews with thirty three servicemen, four of whom were later exposed as unreliable by Neil Sheehan in a New York Times book review. Leaders of VVAW feared involvement with Lane would tarnish the credibility of the WSI. A new steering committee was created without Lane, but he was allowed to continue with fundraising events and was available for legal counsel. The new six-member steering committee for WSI was composed of three national office leaders (Al Hubbard, Craig Scott Moore and Mike Oliver) and three members of the growing list of chapters (Art Flesch, Tim Butz, and William F. Crandell). Organizers hoped Lane would keep a low profile during the event, and he did, as he spent his time flying around the country defending his book. Ultimately, the WSI was an event produced by veterans only, without the need of "so-called experts" such as Lane and Fonda.

After the organizers of the national hearings separated into two groups, they each developed their own events. The CCI advanced its plans for a December event in Washington, D.C., while the WSI's new organizers continued with the original plan to hold its hearings in Detroit. The Washington, DC, event would be called the National Veterans Inquiry. The Detroit event would be called the Winter Soldier Investigation. Seven of the 142 total participants would provide testimony at both events.

The support of antiwar celebrities was considered crucial to generate both money and publicity. A series of benefit productions, Acting in Concert for Peace, were created and featured performances by Jane Fonda, Dick Gregory, Donald Sutherland and Barbara Dane. Two concerts by Crosby and Nash, as well as folk singer Phil Ochs, also raised funds.

The WSI also relied on considerable support from the Detroit community. Dean A. Robb and Ernie Goodman solicited donations from their fellow local attorneys and several clergymen arranged housing for the witnesses. In the words of the Director of Missions for the Detroit Metropolitan Council of Churches, Dr. John B Forsyth, "It is important that the public realize that American atrocities in Vietnam are an every day [sic] occurrence." The Secretary-Treasurer for the United Auto Workers, Emil Mazey and Michigan Secretary of State Richard Austin also helped raise funds for the event.