The British people and their rulers have always been concerned about the possibility of an invasion of their island home by a hostile power striking at them from the European continent. Arthur Balfour, the Prime Minister who did so much to modernize the defense arrangements of his country in the early years of the present century, was especially concerned with this problem of invasion. He called it “This eternal and most important question of our safety against invasion.” In 1908 an entirely new element was introduced into this area of strategic thinking when it was realized, for the first time, that Britain might be invaded from the air. Until this year the British could take comfort from the often quoted words of Admiral Lord St. Vincent to a group of nervous fellow peers at the time of the French invasion danger early in the nineteenth century: “I do not say they cannot come, my Lords, I only say they cannot come by sea.” Now, as a result of unprecedented technological developments it was possible for a vigilant and forward-looking observer to contemplate an air assault upon the United Kingdom that would put Lord St. Vincent's pithy maxim in an entirely new light. Eventually, the fear of air attack assumed a dominating position in the minds of British defense planners; but the origins of this problem have not been studied closely by scholars, even though they deserve attention.