A Mulberry harbour was a British type of temporary harbour developed in World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 to offload cargo on the beaches during the Allied invasion of Normandy.

Two prefabricated or artificial military harbours were taken across the English Channel

English Channel

The English Channel , often referred to simply as the Channel, is an arm of the Atlantic Ocean that separates southern England from northern France, and joins the North Sea to the Atlantic. It is about long and varies in width from at its widest to in the Strait of Dover...

 from Britain with the invading army in sections and assembled off the coast of Normandy

Normandy

Normandy is a geographical region corresponding to the former Duchy of Normandy. It is in France.The continental territory covers 30,627 km² and forms the preponderant part of Normandy and roughly 5% of the territory of France. It is divided for administrative purposes into two régions:...

 as part of the D-Day

D-Day

D-Day is a term often used in military parlance to denote the day on which a combat attack or operation is to be initiated. "D-Day" often represents a variable, designating the day upon which some significant event will occur or has occurred; see Military designation of days and hours for similar...

 invasion of France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 in 1944.
The Dieppe Raid

Dieppe Raid

The Dieppe Raid, also known as the Battle of Dieppe, Operation Rutter or later on Operation Jubilee, during the Second World War, was an Allied attack on the German-occupied port of Dieppe on the northern coast of France on 19 August 1942. The assault began at 5:00 AM and by 10:50 AM the Allied...

 of 1942 had shown that the Allies

Allies

In everyday English usage, allies are people, groups, or nations that have joined together in an association for mutual benefit or to achieve some common purpose, whether or not explicit agreement has been worked out between them...

 could not rely on being able to penetrate the Atlantic Wall

Atlantic Wall

The Atlantic Wall was an extensive system of coastal fortifications built by Nazi Germany between 1942 and 1944 along the western coast of Europe as a defense against an anticipated Allied invasion of the mainland continent from Great Britain.-History:On March 23, 1942 Führer Directive Number 40...

 to capture a port on the north French coast.