Why do weaker states repeatedly commit to wars alongside great powers, even when their involvement is unlikely to change the outcome of the conflict? Ambiguous Alliance offers a fresh explanation of why states join others' conflicts, and what these choices reveal about their foreign policy and grand strategic ambitions. It argues that participation serves as a signal, helping states leverage stronger military commitments from more powerful allies. Michael Cohen's analysis links literatures on signalling, interstate wars, and military alliances to examine which states enter others' fights and why. Drawing on extensive archival sources, he delivers a compelling new study of Australia's commitment to US wars and the US–Australia alliance during the Cold War, alongside cases spanning from the 1750s to the present day. The book also illuminates the prospects of major powers, such as the United States and Russia, to elicit military support from others in contemporary conflicts.

‘Ambiguous Alliance' deftly ties together the literatures on interstate war, alliances, and signalling, offering a novel and compelling explanation for why weak states join major powers in wars they needn't fight. This book is essential reading for those interested in alliances and the understudied US-Australia relationship.'

Michaela Mattes - Professor of Political Science, University of California, Berkeley

‘Cohen brings fresh insight to our understanding of alliances and coalitional warfare. He highlights the puzzling cases of states entering the fray when they could have just as easily stayed out. Why put yourself at risk? For Cohen, smaller states are motivated to fight the wars of bigger states to break the ambiguity of their relationship with the bigger state. Drawing on the under studied case of Australia's security relationship with the United States during the Cold War, he shows that smaller states are more than willing to fight the wars of larger states in the hope that doing so will gain a stronger commitment from the more powerful state.'

Paul Poast - Associate Professor, Department of Political Science, University of Chicago