When NATO leaders convene for the Warsaw summit this July, their agenda will be dominated by external threats—from an aggressive Russia, to the Islamic State of Iraq and al-Sham (ISIS), to the refugee crisis that has spread from the Middle East and North Africa throughout Europe. Yet an equally important issue, though not explicitly on the agenda, will be the internal health of the Alliance. NATO faces unprecedented challenges to its values, cohesion, and effectiveness, all integral to dealing with these external threats.