As military conflicts come to an end, it is not uncommon for societies to expect a "peace dividend" and to engage in elite and popular conversations about how much defense spending is still needed. The issues are similar across countries and time periods: How can defense planners preserve capabilities, avoid the reversibility problem, and plan for the long term? How can they guide the development of technologies and doctrines in a climate of austerity? This manuscript draws lessons from previous historic situations and applies them to today.