 Complete the following phrase:  The channel, which delayed human reoccupation of Great Britain for more than 100,000 years, has in historic times been both an easy entry for seafaring people and a key natural defence, halting invading armies while in conjunction with control of the North Sea allowing Britain to blockade the continent. The most significant failed invasion threats came when the Dutch and Belgian ports were held by a major continental power, e.g. from the Spanish Armada in 1588, Napoleon during the Napoleonic Wars, and Nazi Germany during World War II. Successful invasions include the Roman conquest of Britain and the Norman Conquest in 1066, while the concentration of excellent harbours in the Western Channel on Britains south coast made possible the largest amphibious invasion in history, the Normandy Landings in 1944. Channel naval battles include the Battle of the Downs 1639, 