On 10 May 1940, the day Churchill became Prime Minister, Germany launched an invasion of Belgium, the Netherlands and France, and on 22--23 May the German army reached the English Channel, isolating the British Expeditionary Force.
On 10 May 1940, the day Churchill became Prime Minister, Germany launched an invasion of Belgium, the Netherlands and France, beginning the war in the west in earnest. By 22 and 23 May, after an abortive counterattack at Arras, German forces were pushing towards the English Channel, isolating the British Expeditionary Force.