From 1915, theatrical commentators repeatedly expressed concern over the American ‘Invasion’ of British theatre. For some, the presence of American actors and writers in London was to be celebrated as a sign of a transatlantic theatrical fraternity. Yet, for a large number of critics, it pointed to a more sinister shift in power from which British theatre had to be defended. This chapter examines what these fears reveal about the wider social anxiety of the time, arguing that the perceived threat to the national drama must be understood in dialogue with the perceived threat to national identity that came with the war and the need to defend national honour on the battlefields. Further, it suggests that the concerns raised speak to a growing unease at the changing power dynamic between the two nations and, more acutely, the suspicion levelled at the neutral stance that America adopted in the war. In discussing concerns over the American ‘invasion, this chapter focuses on the growing popularity of the ‘crook’ play. It also looks at the transfer of British productions to America, focussing on Granville Barker’s ground-breaking tour to the east coast as a means of cultural propaganda.