On june 4, 1944, Supreme Allied Commander General Dwight D.
Eisenhower gives the go-ahead for largest amphibious military
operation in history: Operation Overlord, code named D-Day, the
Allied invasion of northern France.

General Dwight D. Eisenhower
By daybreak, 18,000 British and American parachutists were already
on the ground. At 6:30 a.m., American troops came ashore at Utah
and Omaha beaches. At Omaha, the U.S. First Division battled high
seas, mist, mines, burning vehicles-and German coastal batteries,
including an elite infantry division, which spewed heavy fire. Many
wounded Americans ultimately drowned in the high tide. British
divisions, which landed at Gold, Juno, and Sword beaches, and
Canadian troops also met with heavy German fire, but by the end of
the day they were able to push inland.
Despite the German resistance, Allied casualties overall were
relatively light. The United States and Britain each lost about
1,000 men, and Canada 355. Before the day was over, 155,000 Allied
troops would be in Normandy. However, the United States managed to
get only half of the 14,000 vehicles and a quarter of the 14,500
tons of supplies they intended on shore.
Three factors were decisive in the success of the Allied invasion.
First, German counterattacks were firm but sparse, enabling the
Allies to create a broad bridgehead, or advanced position, from
which they were able to build up enormous troop strength. Second,
Allied air cover, which destroyed bridges over the Seine, forced
the Germans to suffer long detours, and naval gunfire proved
decisive in protecting the invasion troops. And third, division and
confusion within the German ranks as to where the invasion would
start and how best to defend their position helped the Allies.
(Hitler, convinced another invasion was coming the next day east of
the Seine River, refused to allow reserves to be pulled from that
area.)
Field Marshal Bernard Montgomery, commander of Britain's
Twenty-first Army Group (but under the overall command of General
Eisenhower, for whom Montgomery, and his ego, proved a perennial
thorn in the side), often claimed later that the invasion had come
off exactly as planned. That was a boast, as evidenced by the
failure to take Caen on the first day, as scheduled. While the
operation was a decided success, considering the number of troops
put ashore and light casualties, improvisation by courageous and
quick-witted commanders also played an enormous role.