Does diplomacy affect the prospects of international conflict and cooperation? Systematic empirical assessment has been hindered by the inferential challenges of separating diplomacy from the distribution of power and interests that underlies its conduct. This paper addresses the question of diplomacy's efficacy by examining the intragovernmental politics of US foreign policy, and the varying influence of diplomatic personnel in the policy process. I claim that diplomats hold the strongest preferences for cooperative relations with their host countries, relative to other participants in the foreign policy process. They also exert substantial influence over the formation and implementation of US policies toward their host countries but their influence is intermittently weakened by the short-term shock of an ambassadorial turnover. As a result, when ambassadors are removed from post, diplomacy is more likely to be eschewed for more conflictual means of settling international disagreements, and opportunities for economic exchange are less likely to be realized. I test this theory using newly collected data on US diplomatic representation, for the global sample of countries from 1960 through 2014. To address concerns of diplomatic staffing being endogenous to political interests, I leverage a natural experiment arising from the State Department's three-year ambassadorial rotation system. The turnover of a US ambassador causes a decrease in US exports to the country experiencing the turnover, and heightens the risk of onset of a militarized dispute between that country and the US. These findings point to bureaucratic delegation as an important but overlooked determinant of macro-level international outcomes.