In this review article, our task is two-fold. First, we will create an overview of how counterfactual history has been understood and defined by historians and scholars from neighbouring fields. The stances vary from completely dismissive to arguments that counterfactual thinking is in fact one of the key methods of any historical enquiry. Second, as an example of counterfactual fiction, we will examine how a notable British tradition of invasion scare fictions has imagined the idea of the islands being taken over by hostile actors, historical or imaginary.