Where is the US–Japan security alliance headed? With the US run by President Donald Trump, this is – at least so it seems for now – not always easy to predict in view of Trump’s frequent pathological mood swings and contradictory policies. For now, at least, Trump and Japanese Prime Minister Shinzo Abe get along just fine and provide each other with what they want from each other – US guarantees to protect the Japanese-controlled Senkaku Islands in the East China Sea in return for Abe not criticizing Trump’s ill-fated isolationist policies in the Asia-Pacific announced by tweets in the early hours of the day. Day-to-day US–Japan alliance management business in the meantime goes on undeterred and helped by recent development and changes on Japan’s security and defence policy agenda, Washington and Tokyo are on a very well-defined path to increase alliance interoperability, aimed at among others transforming the bilateral alliance from asymmetrical to more symmetrical and equal. The revised version of the US–Japan defence guidelines adopted in 2015 in particular are a fundamental step towards increasing Japanese military contributions in the case of a regional contingency with US involvement. The details, challenges and prospects of increased and deeper US–Japan military cooperation in the context of their bilateral alliance will be analysed in this chapter.