At 11 a.m. on Nov. 11, 1918, Winston Churchill was looking out of the window of his Ministry of Munitions, toward Trafalgar Square in London, waiting for the great bell of Big Ben to sound, telling Britons that World War I was finally over. 

“And then suddenly the first stroke of the chime,” he recalled in his war memoirs. “I looked again at the broad street beneath me. It was deserted. From the portals of one of the hotels absorbed by government departments darted the slight figure of a girl clerk, distractedly gesticulating while another stroke of Big Ben resounded. Then from all sides men and women came scurrying into the street. Streams of people poured out of all the buildings. The bells of London began to clash.”

“Flags appeared as if by magic,” he went on. “Swarms of men and women flowed from the Embankment. They mingled with the torrents pouring down the Strand on their way to acclaim the king.”

The happy crowds finally knew for certain that one existential threat to the British Empire — German militarism — had been extinguished, albeit at the cost of nearly one million of its citizens. Churchill, however, knew that the war had also thrown up a large number of new threats, foreign and domestic, that together could still overwhelm his country and its empire. Through much of 1919, he started planning for how to deal with them.