The Allied Expeditionary Force was the official name for the combined armies, navies and air forces of the 26 nations who fought against the Axis Powers in World War II, including the United States, the United Kingdom, the Soviet Union, China, France, the Netherlands, and Poland.
Although each nation's forces had conducted several major campaigns during the war, the first major operation executed by the Allied Expeditionary Force was the invasion of Nazi occupied France along the beaches at Normandy. Together, the Allies continued on from their beachhead at Normandy to liberate Paris and the rest of occupied France as well as other nations dominated by the Third Reich, before pushing their way through Germany to sack Berlin, ousting the Nazis from power.
This is a famous letter that was sent to the "troops of the Allied Expeditionary Forces" who were about to participate in the D-Day invasion of France:
You are about to embark upon the Great Crusade, toward which we have striven these many months. The eyes of the world are upon you. The hopes and prayers of liberty-loving people everywhere march with you. In company with our brave Allies and brothers-in-arms on other Fronts you will bring about the destruction of the German war machine, the elimination of Nazi tyranny over oppressed peoples of Europe, and security for ourselves in a free world.
Your task will not be an easy one. Your enemy is well trained, well equipped and battle-hardened. He will fight savagely.
But this is the year 1944! Much has happened since the Nazi triumphs of 1940-41. The United Nations have inflicted upon the Germans great defeats, in open battle, man-to-man. Our air offensive has seriously reduced their strength in the air and their capacity to wage war on the ground. Our Home Fronts have given us an overwhelming superiority in weapons and munitions of war, and placed at our disposal great reserves of trained fighting men. The tide has turned! The free men of the world are marching together to Victory!
I have full confidence in your courage, devotion to duty and skill in battle. We will accept nothing less than full victory!
Good Luck! And let us all beseech the blessing of Almighty God upon this great and noble undertaking.
June 6, 1944
-- General Dwight D. Eisenhower, supreme commander, Allied Expeditionary Forces, Europe