This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book covers the challenges that Mussolini's aggression created for advocates of collective security. It explains that senior French politicians and diplomats had grown increasingly distressed at a lack of British comprehension of French security concerns prompted by a resurgent, openly revisionist Nazi Germany. The book shows that aspects of the Canadian debate echoed the American one, albeit with distinctive elements given Canada's French and English political debates and its legacy as a Dominion of the United Kingdom. It shows that Japan, Germany's future ally during the Second World War, had a much different relationship with Ethiopia than the other Powers. The collision of empires through the international crisis deriving from Italy's national colonial war tilted the balance of power in favour of the revisionist powers and represented an exceptionally important erosion of the chances to avert a new world war.