What causes variation in the foreign policies of U.S. allies regarding their desired U.S. military role in their region and their troop commitments to U.S. military interventions? This paper addresses this question through documenting and explaining the sources of variation in Australia's foreign policies regarding these issues over four decades. Treating the two major political parties in Australia and their respective leaders who self-select into them as endogenous, the paper argues that Australian foreign policy, whilst always supportive of the U.S. alliance, has systematically varied. This variation has correlated with the political party in power while the late Cold War and post-Cold War balances of power remained constant. While the Labor party has only been willing to send combat troops to large U.S. military interventions when the latter have a supporting United Nations Security Council Resolution, the conservative Liberal party has been willing to military intervene without this multilateral support. The Labor party, unlike the Liberal party, has also frequently proposed the formation and consolidation of multilateral regional institutions. These preferences render the U.S. to have been necessary for the Labor Party but sufficient for the Liberal party. Future Sino-U.S. armed conflict would provide a harder test of these hypotheses.