Gentile enlisted in the US Army in 1975 and graduated from UC-Berkeley's ROTC program in 1986. He completed a PhD in history at Stanford University in 2000.^[3] He served two tours in Iraq, first as the executive officer of a combat brigade in Tikrit in 2003^[4] and then as a squadron commander in western Baghdad in 2006.^[5]

Gentile's post-graduate academic work was on the topic of military air power. How Effective is Strategic Bombing?, published in 2001, challenged the conclusions of the Strategic Bombing Survey. Reflecting on Gentile’s work, the book review editor for The Journal of Conflict Studies wrote that “strategic bombing seems to have adapted itself nicely to the exigencies of democratic government; providing a way of waging limited war at arm's length, minimizing casualties on both sides of the conflict, and satisfying both domestic population and politician.” But that “it is left to Gian Gentile ... to pose the question US policy-makers should be asking: How effective is strategic bombing?” The reviewer opined that “Gentile's answers are fresh because he ... show[s] the reader that the question has rarely been answered honestly or even, in some cases, competently.” He also echoed Gentile’s central point that “the US Air Force among others has frequently, and sometimes purposely, failed to distinguish between the effects of strategic bombing and its effectiveness,” emphasizing that the “effects, physically observed and measured, are relatively easy to see and to report--and impress the public with.” However, “the effectiveness of same is wide open for debate.”^[6]

Gentile's second book, Wrong Turn: America's Deadly Embrace of Counterinsurgency, appeared in July 2013.^[7] Andrew Rosenbaum, in the New York Journal of Books, said: "Col. Gentile very capably shows that counterinsurgency, which can be roughly summed up as a 'nation-building' strategy, didn’t work in the past, when the British tried it in Malaysia, nor when the U.S. tried it in Vietnam, and that it certainly did not work in Iraq and Afghanistan, despite the hype it received when it was tried. A separate chapter is dedicated to each of these efforts, and Col. Gentile goes into sufficient detail to make a very good case. Col. Gentile then shows that the strategy is fundamentally flawed, and cannot work in the form that it has been proposed."^[8]

Gentile is a prominent critic of the U.S. military's use of counter-insurgency.^[9][10] He believes that the 2007 surge was not the primary cause of the reduction in violence in Iraq and that effective counter-insurgency tactics were practiced by American troops in Iraq starting in 2004, rather than being introduced in 2007.^[9][11][12] Instead, Gentile argues that paying Sunni insurgents to help coalition forces eradicate al-Qaeda in Iraq and Muqtada al-Sadr's decision to call a cease fire in southern Iraq were the main causal factors.^[13] He further argues that the U.S. military is now concentrating excessively on counter-insurgency, to the detriment of its capacity to fight conventional wars.^[14] Following Andrew Bacevich, Gentile believes that the prominence of counterinsurgency has led to an unrealistic view of the American military's power and capacity to change the world.^[11]