Basing its foreign policy on the Wilsonian internationalism, the new Turkish Republic established good relations with countries around the world. It signed neutrality and friendship treaties, and pursued a neutral foreign policy. However, at the end of World War II, it abandoned its longtime neutral foreign policy and aimed to establish closer ties with the American-led West. This thesis examines how and why Turkey shifted its foreign policy from neutrality to active alliance. In the first half of the thesis, I closely deal with what role international developments played in that shift. First, I focus on how Josef Stalin's efforts to obtain bases and joint-control with Turkey over the Turkish Straits created a threat to Turkey's national security. Then, I explore how this threat forced Turkey to leave its neutral foreign policy and seek closer ties with the U.S. In the second half of the thesis, I examine how Turkey's search for economic aid and military commitment accelerated and intensified the shift from neutrality to active alliance. First, I focus on how Turkish officials aggressively sought economic assistance from the U.S. and how U.S. officials became resistant to the Turkish requests for additional aid beginning with the second half of 1947. Considering that Turkey was less vulnerable to the Soviet threat, U.S. officials judged that Turkey did not need aid as much as Western Europe did. In order to overcome the resistance, Turkish officials exaggerated the Soviet threat and used the problem of high defense spending. Then, I explore how Turkish officials sought a military commitment from the U.S. A U.S. military commitment could alleviate the problem of high defense spending and facilitate the flow of economic aid from the U.S. Therefore, Turkish officials carried on a diplomatic offensive to secure a military commitment from the U.S. In doing so, they distanced themselves from neutrality and became an institutional ally of the U.S. in 1952. / History