Military assistance has a bad reputation. Large-scale attempts to build partner militaries in Afghanistan, Iraq, and Vietnam cost the United States billions of dollars and ended ignominiously, with the collapse of local forces as American troops withdrew. Arms transfers of sophisticated, American-made weapons often appear to do more harm than good. Yet military assistance and support—operating indirectly through partners—when done right, can deliver remarkable strategic results for the United States and its partners. How to work effectively with partner militaries is one of the most pressing national security challenges for the United States today. Richard Bennet and Alexander Noyes offer a systematic look at military assistance in the twenty-first century, examining a frequently deployed but often misunderstood set of tools that allows the United States to leverage partner militaries to achieve national security objectives. Bennet and Noyes posit that two main factors—the degree of interest alignment on security issues and the level of institutional capacity of the receiving force—will be the most important variables in Washington’s ability to build militarily effective partners.