Between May and September of 1940 Germany planned an invasion of Britain by sea and by air. Having overrun much of Europe, and with Holland, Belgium, and France all capitulating in the late spring of that year, the Nazi machine had proven its efficiency and its fearsomeness. Britain was alone—this was eighteen months before America came into the war—and, following the evacuation from the beaches of Dunkirk in late May and early June, she was severely under-equipped. As Leo McKinstry explains in this immaculately researched and compellingly written book, the evacuation delivered 338,000 men back to England but left behind “2,472 guns, 615 tanks, 63,870 vehicles, 90,000 rifles, 7,000 tons of ammunition, 165,000 tons of petrol, 20,000 motorcycles, and vast quantities of other equipment.” That alone would be enough to stop an invading army in its tracks, and even the Germans were astonished by how much matériel the fleeing British Expeditionary Force left behind. Now, Britain had to go into overdrive not just to turn its routed soldiers into an effective home army, but also to produce the weapons and machinery for them to use when the Wehrmacht appeared.

Yet the full-scale German invasion never materialized. McKinstry’s book explains why and how the myths about British incompetence and amateurishness in the face of this lethal threat turned out to be largely untrue. Operation Sea Lion—the name the Germans gave to their idea of invading England—was to take place in mid-September, but Britain feared it would come in