Military innovation literature touts new technologies and the adaptation that militaries make to successfully innovate. Military effectiveness literature examines how to make militaries more successful in combat. But many militaries spend years between wars, often without a clear understanding of the next major security threat. This dissertation examines how militaries can best spend limited budgets to most effectively prepare for rapid expansion. My central claim is that militaries that understand the state of the battlefield who update doctrine and spend effort to educate and train officers will be better prepared to more rapidly expand. Modern warfare is difficult, and each new conflict introduces novel technologies that must be integrated well and defended against to succeed in future combat. However, political constraints remain the same: politicians rarely seek to spend limited peacetime budgets preparing for war. Given the challenge of modern warfare, effective militaries have a clear and appropriate conception of what they want to do on the battlefield and can train and test officers and men in what they need to do. Therefore, building an effective peacetime officer corps focused on the central tasks for major peer warfare should be the single greatest priority. I test this theory using three historical case studies. For ease of comparison, I use the American Army, British Army, and German Army during the Interwar Period. Both the American and German Armies prioritized protecting their officer corps which could then form the kernel of a larger army when called upon. On the other hand, the British Army did not invest scarce resources into training and educating the officer corps. The findings help to understand the central puzzle of how militaries that are undermanned, underequipped, and undertrained can emerge successfully from periods without large scale combat. Variation in preparation can help to better understand why some militaries succeed and some fail on the battlefield. For policymakers, and military leaders, this dissertation suggests how to manage periods of budget scarcity, while still preparing for war.