[4][6][13] Through the work of the PNAC, from 1998, Kagan was an early and strong advocate of military action in Syria, Iran, Afghanistan as well as to "remove Mr. Hussein and his regime from power.

[17][18][19] During the 2008 presidential campaign he served as foreign policy advisor to John McCain, the Republican Party's nominee for President of the United States in the 2008 election.

"[24] In 2008, Kagan wrote an article titled "Neocon Nation: Neoconservatism, c. 1776" for World Affairs, describing the main components of American neoconservatism as a belief in the rectitude of applying US moralism to the world stage, support for the US to act alone, the promotion of American-style liberty and democracy in other countries, the belief in American hegemony,[25] the confidence in US military power, and a distrust of international institutions.

"[28][29] In a February 2017 essay for Foreign Policy, Kagan argued that U.S. post-Cold War retrenchment in global affairs has emboldened Russia and China, "the two great revisionist powers," and will eventually lead to instability and conflict.

[12] He has also written for The New York Times, Foreign Affairs, The Wall Street Journal, Commentary, World Affairs, and Policy Review.In 2003, Kagan's book Of Paradise and Power: America and Europe in the New World Order, published on the eve of the US invasion of Iraq, created something of a sensation through its assertions that Europeans tended to favor peaceful resolutions of international disputes while the United States takes a more "Hobbesian" view in which certain kinds of disagreement can only be settled by force, or, as he put it: "Americans are from Mars and Europe is from Venus."

A New York Times book reviewer, Ivo H. Daalder wrote: When it comes to setting national priorities, determining threats, defining challenges, and fashioning and implementing foreign and defense policies, the United States and Europe have parted ways, writes Mr. Kagan, concluding, in words already famous in another context, 'Americans are from Mars and Europeans are from Venus.

'[32]In Dangerous Nation: America's Place in the World from its Earliest Days to the Dawn of the Twentieth Century (2006) Kagan argued forcefully against what he considers the widespread misconception that the United States had been isolationist since its inception.

[33] Kagan's essay "Not Fade Away: The Myth of American Decline" (The New Republic, February 2, 2012)[34] was very positively received by President Obama.

"[39] In September 2021, Kagan wrote a related opinion essay published in The Washington Post by the title "Our constitutional crisis is already here".

"[41] In October of 2024, he resigned as editor-at-large from the Post due to its decision not to endorse a candidate in the 2024 United States presidential election between Trump and Kamala Harris.