The National Defense Panel’s (NDP’s) independent assessment of the 2014 Quadrennial Defense Review (QDR) raises once again the question of America’s ability to fight and win simultaneous conflicts, a thorny conundrum in a time of strong pressure on defense budgets.1 Long a mainstay of American defense strategy, the “two-war construct” remained an explicit commitment until the Clinton administration. Subsequently, both Democratic and Republican administrations began to parse the requirement more ambiguously as the size of the military fell. The Clinton administration saw the first iteration of the “win–hold–win” formulation. The George W. Bush national security team articulated a requirement to “maintain the capability to defeat any attempt by an enemy—whether a state or nonstate actor—to impose its will on the United States, our allies or our friends.”2 This language was further relaxed in the Obama administration, which opined,     If deterrence fails at any given time, U.S. forces will be capable of defeating a regional adversary in a large-scale multiphased campaign, and denying the objectives of—or imposing unacceptable      costs on—a second aggressor in another region.3This may seem merely a question of semantics, but the difference is marked. The 2014 QDR language signals a projected lack of capacity to do in the second scenario what is explicitly described in the first. The 2015 National Security Strategy is similarly imprecise, stating that “if deterrence fails, U.S. forces will be ready to project power globally to defeat and deny aggression in multiple theaters.”4 An ability to “fight and win two wars simultaneously,” once enshrined in public strategic documents, sends an unambiguous signal that the United States intends to maintain military capabilities able to meet this clear requirement. For decades this commitment underpinned our alliance structure worldwide, reassuring friends and partners and deterring potential adversaries. The qualifying language of the last several administrations signals something less.5The “two-war construct,” as the NDP phrases it, is imperative for reasons that are fairly simple to explain. The United States bases its national security on an extensive network of alliances and bilateral defense arrangements. As Michael O’Hanlon has pointed out, 2     The United States leads a global alliance system of more than 60 partner states that collectively account for almost 80 percent of global [gross domestic product] and more than 80 percent of          global military spending between them.6This system, which provides forward basing, overflight rights, political legitimacy and additional military forces in time of conflict, is of inestimable value, but its viability as well as its deterrent effect hinges on American credibility. Our allies will be with us—if they know we will be there for them. Should the United States find itself committed to one major theater war (for example, on the Korean peninsula) but unable to respond decisively in another (say, in the Middle East or on NATO’s borders), then that credibility is compromised not just at the point of collision but everywhere. Put another way, an America able to intervene decisively in only one region of the world at a time is arguably no longer a global power. So constrained, the United States will find it difficult to play its historical role as a guarantor of a stable global system, a role whose net effect has been to bring into being, largely if not entirely through America’s own efforts, a rules-based international and economic order that has widely benefited much of the world.7This is why the NDP noted, “We find . . . the two-war construct to be as powerful as ever.”8 Fighting two wars at once is never desirable. But as the only superpower, and with the nation’s security and economic well-being invested in a stable international order, America cannot fully control what lands on its plate. We might hope to fight no more than one war at a time. If we are wrong and we cannot cope, the extensive network of alliances and partnerships established over decades—a core and vital interest—is in grave danger.