He then taught for a number of years at the School of Advanced International Studies (part of Johns Hopkins University in Washington, D.C.).
[1] Appointed to be Ambassador to the OAS by President Harry S. Truman, he served from January 2, 1951 until November 12, 1960.
[2] Dreier primarily saw the OAS as a way to limit the spread of communism in Latin America during the Cold War, with his tenure including events such as the 1954 Guatemalan coup d'état and the Cuban Revolution.
Furthermore, he believed that the OAS needed to address social and economic concerns in order to succeed.
After retirement, he played an active role as a teacher and trustee of the College of the Atlantic in Bar Harbor, Maine, and was involved in conservation projects.