The British Empire died on the Somme on July 1st 1916. 60,000 casualties. 20,000 dead. In one day. For a few hundred yards of mud. It was the worst day of the Great War, but not unusual in it's losses compared to it's gains. In comparison, in the Battle of Waterloo of 1815, considered by many the greatest battle of the 19th century & Britain's greatest battle ever (to-date), the Empire suffered about 1,500 dead. It's Belgian, Dutch & Prussian allies lost another 2,000 dead.
Before the Somme campaign was over Britain endured nearly 500,000 dead, wounded & captured. The French, Germans & Russians lost far more. The numbers boggle the mind.
The 1914-1918 war saw Britain spend away two centuries of unprecedented national wealth. In addition to suffering 900,000 dead. In a war that was supposed to last 4 weeks (aren't they all?).
Before the war, the idea of Empire was a source of national pride. After 1918, it was a cynical joke. It's a miracle the Empire lasted another 4 decades.
In contrast, the United States experience in World War One was the opening act to the American Century.