The US frequently forms coalitions to go to war with other countries and Turkey is one of its many allies asked to contribute to these military coalitions. This thesis aims to reveal why Turkey joins some US-led military coalitions but chooses not to join others. In order to achieve this aim, this study identified eight historical points, called decision occasions, when decisions on whether or not to join coalitions were made, and analyzed seven independent variables for each of these occasions utilizing structured, focused comparison analysis. These decision occasions were August 1990, December 1990 and January 1991 from the Gulf War; November 2001 and January 2002 from the Afghanistan War; March 1, 2003, March 20, 2003 and October 2003 from the Iraq War. The study used paired comparison analysis carried out with decision-makers involved with each decision occasion, together with 47 elite interviews conducted with cabinet members, parliamentarians, high level civilian and military bureaucrats to rank the independent variables. The findings indicate that the decision-makers regard alliance dependence on the US as a key factor in Turkey’s joining coalitions; and the actual decision-making process and risk aversion as the main factors in Turkey’s decision not to join coalitions. Legitimacy by UNSC resolution and NATO decision, along with Turkey’s interest in the region/desire to enhance influence did not produce any consistent pattern; yet, the decision-makers viewed them as supportive factors in the context of joining the coalition. Finally, the seventh variable, military capability is not regarded as influential in any decisions made to join or not to join a US-led military coalition.