America declared war on Germany on 6 April 1917, tipping the balance in favour of Britain, France and their Allies. The USA had resisted getting embroiled in the war in Europe for almost three years, but after the declaration of war, it sent troops to fight on the Western Front in France and Belgium.

Photo: Postcard of Joseph "Black Jack" Pershing (Centre - R), the US Army General who led the American Expeditionary Force in World War I, being welcomed in Boulogne, northern France, by French General Peltier . Credit: AFP PHOTO / Historial de Péronne