“I never felt that the exact dating was a worthwhile question. More important was the “why” question.”
Sometimes the date explains the “why”; Russia’s surrender to the Central Powers in 1917 led the US to get into the war the same year (after our president campaigned on the slogan “He kept us out of war” in 1916). The Battle of Hastings occurred while Viking influence was waning; before battling William, Harry had to double-time south to meet him after defeating a Viking army at Stamford Bridge.
Nicholas II abdicated in March 1917, shortly before Wilson asked Congress for a declaration of war against Germany, but the Russians stayed in the war until after the Bolshevik takeover in November 1917. Lenin's treaty with Germany was in the spring of 1918.