I continue my quick journey through some key events of the First World War.
The USA declared war on Germany on the 6th of April 1917. America was brought into the war by Germany's declaration of unrestricted submarine warfare in January of that year. This meant that German submarines were authorised to sink any ship without warning if they thought it might be carrying goods to one of Germany's enemies (previously the quaint rules of war meant that submarines had to surface and send boarding parties to search ships for war goods); Germany's hope was that unrestricted submarine warfare could knock Britain out of the war. US opinion was also turned against Germany by the revelation that it was attempting to build an anti-American alliance with Mexico.
When America declared war on Germany it had a tiny army and it would clearly be some time before it could raise and train forces that could be sent to fight in Europe. But the German leaders would have known that the USA's declaration of war had set the clock ticking. The Americans had a bottomless reservoir of manpower and it was just a matter of time before the USA was ready to send a vast force to France. If Germany was to have any chance of winning the war, it had to do so before the American armies arrived. And when it became apparent that unrestricted submarine warfare was not bringing Britain to its knees, Germany's leaders realised that they would have to win the war on land.
The declaration of war on Germany also marked the moment when the USA became a world power.