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"Ground War in Iran Would Be Hell": Brandan Buck's 600,000 to 1.6 Million

In March 2026, as the Pentagon began preparing ground options, a Cato Institute scholar published the most widely cited American estimate of what an invasion of Iran would take. Brandan Buck scaled two real battles, the 2003 invasion of Iraq and the 2004 fight for Fallujah, to Iran's size. He landed between 600,000 and 1.6 million American troops, and called it "the largest U.S. military undertaking since the Second World War."

Evidence rating: CONFIRMED that Buck published the estimate in UnHerd, republished by the Cato Institute, and was quoted with the 1.6 million figure by Common Dreams on 26 March 2026.

Who he is​

Brandan Buck is a scholar at the Cato Institute, a libertarian think tank in Washington that favors a restrained foreign policy. His essay, "Ground War in Iran Would Be Hell," ran in the British magazine UnHerd and was republished by Cato (UnHerd, March 2026; Cato).

How he got his numbers​

Start with the country. Iran is nearly four times the size of Iraq, with more than three times the population. Mountains cover much of it. Tehran alone holds more than 15 million people.

Method one: Fallujah density. The 2004 battles for Fallujah, Iraq, took a high ratio of American troops to residents to clear and hold one city. Applied to Iran, Buck arrives at more than 600,000 troops, roughly the size of the American force in Vietnam at its peak.

Method two: scale Iraq. The 2003 invasion of Iraq used about 150,000 American ground troops in the invasion force. Scaled by Iran's population, the number becomes "as many as 1.6 million troops" (as quoted by Common Dreams, 26 Mar 2026).

His conclusion, as Common Dreams reported it: an invasion "would likely rival or exceed the scale of Vietnam or the 1991 Gulf War," amounting to "the largest U.S. military undertaking since the Second World War."

How it compares​

  • The Jerusalem Post column, 30 August 2026: "between 750 thousand and 1.5 million American troops." Almost exactly Buck's range, arrived at five months later in an Israeli newspaper, with a plan to recruit indebted young Americans. See the Jerusalem Post column.
  • Kenneth Pollack, Foreign Affairs, 18 March 2026: securing Iran after a collapse "would require well over half a million troops, at least initially" (a197).
  • Brookings, 2009: 1.4 million to occupy. Army staff college thesis, 2007: about 1.25 million. See force-planning documents.
  • White House adviser David Sacks, June 2026: warned an invasion would be a "suicide mission," requiring, as Drop Site News paraphrased him, "possibly a million soldiers" (Drop Site News on X, 20 Jun 2026).
  • Col. Douglas Macgregor (ret.), 8 April 2026: "Short of a nuclear strike or a million soldiers properly organized, trained and equipped for a new kind of warfare, victory over Iran is virtually impossible" (X).

Libertarian, centrist, Army, Israeli, and White House voices all land in the same band: half a million to one and a half million.

Why it matters​

The US active-duty Army is about 452,000 soldiers. Buck's low number is larger than the whole Army; his high number is more than three times it. A war on that scale cannot be staffed by volunteers. The only ways to fill it are a massive recall of reservists and veterans, a draft, or, as the Jerusalem Post column proposed, buying recruits with debt relief. See divisions needed versus divisions available and Reserves, the Guard, and the road to a draft.

Status: Brandan Buck is alive. Kenneth Pollack is alive. David Sacks is alive. Douglas Macgregor is alive.

Last Updated: 2026-09-26

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