The title and opening paragraph of this book deliberately evoke Sun Tzu and Clausewitz, setting a high bar the author can be forgiven for not clearing. Yet, the title is also misleading, as The Science of War, which combines the modesty of an experienced military analyst with the hubris of an American strategic planner, presents little that is recognizably scientific.

Michael E. O'Hanlon, perhaps the most visible and prolific defense intellectual in the United States, seeks to lay out “a structured, analytical, often quantitative, often rather technical side to preparing for combat.” This task is both worthwhile and frightfully difficult. He makes explicit the massive uncertainties inherent to military preparation and conflict, yet still claims that while “recognizing scientific methods in defense analysis to be imprecise, we must nonetheless strive to understand, improve, and employ them” (pp. 1–2).

Following this convincing if uncontroversial case for analytical rigor in military matters, the book delivers four substantive chapters on defense analysis with an American focus. Each generally opens with thematic questions: “Does the United States spend too much on defense?” for Chapter 1; “Can we make meaningful estimates of what force package would be adequate to prevail decisively in a proposed conflict?” for Chapter 2. Chapter 3, on logistics and basing, contains no questions but delivers the largest amount of valuable, concrete information not readily found elsewhere. Chapter 4 asks how “analysts wade into the complex world of actual military science,” such as physics and engineering. Each chapter employs hypothetical scenarios, such as a Chinese invasion of Taiwan, to show how one can apply the analytical techniques.

Two different audiences will find this book of interest. The first is the intended one, those seeking an introduction to the ins and outs of military planning. There are plenty of useful facts in the book, and even more useful references and appendices. I learned much from both its many anecdotes—tritium can disguise a nuclear test's yield—and rules of thumb—90% of all missiles fired at unarmed ships find their target. The reader gains some insight on how the sausage of U.S. military power is made through the acronym-intensive world of FYDPs (Future Years Defense Programs), TPFDLs (Time-Phased Force and Deployment Lists), and FOBs (Forward Operating Bases). The book depicts the awe-inspiring logistics necessary to deploy an air wing halfway around the world, and the technical hurdles that must be overcome to produce anything approaching a viable ballistic missile defense.

O'Hanlon's interpretation of what science entails is questionable, however. Much of the book's claim to the label rests on the use of basic math to calculate times required to deploy military units, resources spent on certain missions and regions, and force sizes needed to achieve conflict outcomes. These models are obviously valuable tools, but their inconsistent, unsystematic, and atheoretical use is troubling.

Consider the book jacket's claim that O'Hanlon's models predicting “higher-than-anticipated troop fatalities in Iraq … have since been vindicated.” This is wrong. O'Hanlon assumed a short but intense urban fight (rather than a long insurgency) to get a casualty estimate ranging from 650 to 3,000, and he claims that “[m]y instincts about how the war might go were largely wrong, but by being forced to follow the rigor of the model, I wound up with relatively accurate results nonetheless” (p. 83). This is not science. His theory (i.e., “instincts”) was wrong—never mind that U.S. and allied casualties in Iraq have exceeded the upper bound by 60%.

Perhaps being right for the wrong reasons is not a fatal flaw. Geocentric conceptions of the universe do a terrific job of predicting astral movements, despite the sun's not revolving around the Earth. Celestial navigation may no longer merit the term “science,” but still works quite well as a tool. But the book displays an endogeneity—where outcomes determine inputs—threatening to make these models not only suspect but also dangerous.

Unsurprisingly, these models cannot explain everything. For example, O'Hanlon poses the question “Why do evenly matched militaries sometimes fight to a draw—and sometimes not?” The difference ends up being “a brilliant plan,” such as the one pursued by Germany against France in 1940. The author adjusts the assumed parameters of the model (defensive advantage equals zero, surprise equals two, etc.) to get the desired lopsided result, rather than showing how one determines ex ante values that might capture a plan (pp. 120–22).

This endogeneity leads to the second, perhaps unintended, audience who will profit from this book: those seeking a paradigmatic example of an American strategic planner's expansive view on feasible uses for the U.S. military. For even the most ambitious missions, O'Hanlon's models predict lavish uses of resources that the United States can afford (materiel) and whittles down the resources with which it is less prodigal (personnel). One calculation leads the book to observe, almost offhandedly, that keeping the Straits of Hormuz “would probably not require more than twenty submarines” to counter Iran's three submarine navy, a stunning flotilla within the means of the United States and no other state (p. 123). He notes that the United States “only [my emphasis] has about fifteen major air bases abroad in a total of ten countries now” (p. 158).

At other times, O'Hanlon does not follow through on calculations hitting limits even the United States must observe. In the Hormuz scenario, he cites the need for ballistic missile defense ships along every 50 miles of Iranian coastline. By my math, this would require 30 ships at any given time. The United States possesses only 18 ships with this capability. The actual number required, given rotation schedules, exceeds all of the Aegis platforms (the only ships capable of such an upgrade) currently in the fleet.

Worse still, as with the 1940 case, when the historically based rules of thumb do not produce the desired hypothetical results, O'Hanlon adjusts. Modeling a U.S.-led stabilization of a collapsing Pakistan—based on best-case assumptions verging on the fantastic (i.e., Pakistan resembles post–World War II Japan)—he predicts a minimum requirement of a million soldiers. Yet, somehow (these calculations are absent) he gets down to 100,000–200,000 troops, perhaps coincidentally the maximum number the United States has been able to muster for its ongoing conflicts (pp. 109–10). This number is apparently also sufficient to “secure” Congo or Indonesia.

In a sense, these analyses support O'Hanlon's central claim regarding these techniques' usefulness. The math's simplicity lays bare the fact that outcomes rest entirely on assumptions; as computer programmers know well, “garbage in, garbage out.” Domestic politics, ideology, and wishful thinking appear to do a much better job of explaining what numbers are spit out than the actual workings of the model. Poor or, perhaps worse, flexible assumptions, including the author's, can produce dangerous guidance for policymakers. Under uncertainty, the models are likely to produce answers that the powerful want to hear. This tendency is compounded by the book's title. Calling something a “science” places its claims in a privileged position relative to other analyses. But this book does not present a science of war so much as the art of (to use another military acronym) the SWAG.

As an introduction to a vital realm of analysis, this book has much to offer. But its greater value may be as a primary text describing the type of military thinking that gets states into trouble. Indeed, a symptom of this pathology is that no rival textbooks exist, and thus The Science of War presents evidence not only of the models' shortcoming but also that of our profession.