Abstract: The 2015 National Military Strategy identifies war with a major power as a growing possibility. The more the United States demonstrates it is willing and able to undertake a big war, the more unlikely it is that it will have to do so. Thus, the US military should undertake analyses, wargames, and exercises focused on rapid expansion of the force, to include creating new formations. ********** After the September 11 terrorist attacks on the United States the focus of the American military shifted quickly and dramatically. Previously, most attention was on quick, high-tempo operations against the conventional forces of rogue states. Using advanced technology and exquisitely trained units, the US military was designed to crush state adversaries in short order. Desert Storm was the prevailing paradigm. (1) After September 11, the US intervention in Afghanistan, and the outbreak of insurgency in Iraq in 2003, the American military quickly shifted to counterinsurgency, counterterrorism, and man hunting. This was a deep and far-ranging change. The human domain of warfare, which had drifted into insignificance during the revolution in military affairs of the 1990s, returned with a vengeance. Conventional forces learned the importance of cultural understanding in counterinsurgency. Special operations forces moved from the periphery to the centerpiece of American military strategy. (2) The military and the intelligence communities fused together to identify opponents and neutralize them. The defense industry provided a massive array of equipment and systems optimized for counterinsurgency and counterterrorism. While this was a dramatically different type of activity than anyone had expected, thought about, and prepared for, the US military adapted on the fly. While the American military was learning to fight extremists, insurgents, and terrorists, conventional war was given little thought and effort. As US involvement in Afghanistan and Iraq subsided, defense officials and military leaders began redefining their focus once again. This has proved difficult. In the past, adversaries--whether the Soviet Union during the Cold War, Saddam Hussein's Iraq, or the Iraqi and Afghan insurgents--drove such reorientations and provided a beacon to guide defense policymakers and military leaders. In the contemporary security environment, there is no predominant adversary. This complicates the military's ongoing reorientation since optimizing for one type of conflict or enemy results in suboptimizing for others. What is clear, though, is that the military must prepare for both irregular or state opponents. As the 2015 National Military Strategy stated: For the past decade, our military campaigns primarily have consisted of operations against violent extremist networks. But today, and into the foreseeable future, we must pay greater attention to challenges posed by state actors ... Today, the probability of US involvement in interstate war with a major power is assessed to be low but growing. (3) This is easy--and important--to say, but tough to do in an increasingly austere resource environment. A future interstate war with a major power would not reprise Desert Storm or the 2003 invasion of Iraq in which the US military overwhelmed enemy forces in lightening campaigns with limited American casualties. Chances are it would be costly and possibly long. As the two world wars showed, major powers sometimes go to war expecting a short conflict--a Franco-Prussian War--only to stumble into a long, bloody slogging match. Even though every American wants to avoid this situation, it is important to consider its possibility. Since the National Military Strategy identifies interstate war with a major power as a growing probability, Americans must ask themselves whether the United States could still fight a conflict lasting years and demanding a major expansion of the armed forces. …