Correct answer:
The invasion at Normandy
Further detail:
The D-Day invasion in World War II gave the Allied forces entry into France. It reopened a Western front in the war with Germany.
The Germans knew that the Allies were planning an advance into France to try to retake the Western front in World War II. The Allies used various forms of deception under "Operation Fortitude" to get the Germans to think they were getting ready to invade in the Pas-de-Calais region in northern France, keeping their actual plans to invade at the beaches of Normandy (in northwest France) a top secret. Troops landed at five sites along the Normandy beaches in the famous "D-Day" invasion of 1944.
"D-day," is actually military jargon for the day any operation goes into action, and H-hour" is the hour at which the operation commences. "Operation Overlord," as the planned incursion of France was called, has become the all-time most famous "D-Day" of American military forces. The Allies were seeking to retake France, which had fallen under German control early in the war.