This paper seeks to understand the real influence that public diplomacy may have over American foreign policy vis-à-vis its effect on public opinion. In order to examine that influence, the paper uses the case of American intervention against the Islamic State in Iraq and al-Sham (ISIS, ISIL, or Dash) to examine how different kinds of elites influence American public opinion through political communication. It tests RM Entman’s Cascading Activation model of elite influence as a framework to understand public diplomacy, replicating his qualitative study of press coverage with specific focus on foreign sources. It also expands the model by testing elites’ respective abilities to directly influence opinion, using a controlled randomized survey experiment. The results of the mixed-method analysis demonstrate that elites from foreign countries, NGOs, and transnational organizations may have greater influence over American foreign policy outcomes than domestic elites outside of the President’s administration. These foreign and transnational elites are relied upon by the press to define the realities of international crises long before the domestic debate on intervention begins, limiting the range of appropriate responses to a crisis. Moreover, the quantitative experiment demonstrated that Iraqi Prime Minister Haider al-Abadi had greater influence on opinion regarding the specifics of intervention policy than President Obama or Senator Harry Reid. These findings indicate that the identity of the matters when attempting to influence American public opinion, and quantitative research may help identify those who are best suited to promote a certain foreign policy goal.