He opposed the Vietnam War and was an outspoken critic of presidents Lyndon B. Johnson, Richard Nixon, and Ronald Reagan and what he viewed as their abuses of presidential power.

[15] His anthology, Documents of American History (1938), reaching its tenth edition (co-edited with his former student Milton Cantor) in 1988, half a century after its first appearance, remains a standard collection work of primary sources.

His two documentary histories, The Blue and the Gray and The Spirit of Seventy-Six (the latter co-edited with his longtime friend and Columbia colleague Richard B. Morris), are comprehensive collections of primary sources on the Civil War and the American Revolution as seen by participants.

With Richard B. Morris, he also co-edited the highly influential New American Nation Series, a multi-volume collaborative history of the United States under whose aegis appeared many significant and prize-winning works of historical scholarship.

At Columbia, Commager mentored a series of distinguished historians who earned their PhD degrees under his tutelage, including Harold Hyman, Leonard W. Levy, and William E. Leuchtenburg.

Although he was skilled at scholarly research and analysis, he preferred to devise and expound sweeping interpretations of historical events and processes, while also making available primary sources so that people could study history for themselves.

Commager was representative of a generation of like-minded historians widely read by the general public, including Samuel Eliot Morison, Allan Nevins, Richard Hofstadter, Arthur Schlesinger Jr., and C. Vann Woodward.

Commager opposed McCarthyism in the 1940s and 1950s, the war in Vietnam (on constitutional grounds), and what he saw as the rampant illegalities and unconstitutionalities perpetrated by the administrations of Richard Nixon and Ronald Reagan.

One favorite cause was his campaign to point out that, because the budget of the Central Intelligence Agency is classified, it violates the requirement of Article One of the Constitution that no moneys can be spent by the federal government except those specifically appropriated by Congress.

[citation needed] Commager wrote hundreds of essays and opinion pieces on history or presenting a historical perspective on current issues for popular magazines and newspapers.

Benjamin W. Cramer states: Commager's lifelong advocacy of intellectual freedom, popular knowledge, and the historical interpretation of contemporary issues has had long-lasting influence on scholars and public advocates, though over the years his politics has been seen as either too liberal or too conservative by various detractors.

In 1953 the NAACP Legal Defense Fund asked Commager for advice for their argument before the Supreme Court for the case of Brown v. Board of Education, but at the time he was not persuaded that this litigation would succeed on historical grounds, and so advised the lawyers.

[18] Commager and his co-author Samuel Eliot Morison received vigorous criticism from African-American intellectuals and other scholars for their popular textbook The Growth of the American Republic, first published in 1930.

Morison refused, however, to remove repeated references to the anti-abolitionist caricature of "Sambo", which he claimed were vital in understanding the racist nature of American culture in the late 19th and early 20th centuries, an era when even the most enlightened progressive thinkers routinely explained many aspects of human behavior as being a result of innate racial or ethnic characteristics.