On 16 July 1940, Germany's leader Adolf Hitler issued the following order, known as Directive Number 16:
"As England, in spite of the hopelessness of her military position, has so far shown herself unwilling to come to any compromise, I have decided to begin to prepare for, and if necessary to carry out, an invasion of England.
"If necessary the island will be occupied."
The word went out to the German army, navy and air force: prepare for the conquest of Britain.
Hitler's instructions came at a point in the course of World War Two where he had achieved domination of almost all of western Europe.
During the previous two months Germany had successfully conquered Holland, Belgium, Luxembourg and France, besides forcing the British army to mount a frantic evacuation from Dunkirk.
Prime Minister Winston Churchill himself acknowledged the likelihood of Nazi Germany now moving against its one remaining enemy.
"The Battle of France is over," he told the House of Commons on 18 June. "I expect that the Battle of Britain is about to begin."
Yet Hitler did not decide to prepare to invade Britain until a full month after the fall of France. It seems he anticipated the British would surrender, or even try to negotiate terms with Germany.
As the historian Andrew Roberts notes, "because Nazi ideology did not call for the invasion of Britain, the Nazis...had failed to plan coherently."
Consequently Hitler's ideas for an invasion, codenamed Operation Sealion, were not the result of long-term research and careful preparation. Rather it was a short-term initiative, rustled up in haste.
During a meeting on 31 July, Hitler learned from his staff of various problems that would at best delay, at worst prevent an invasion of Britain from taking place.
Hitler wanted landings on a wide front from Ramsgate to Lyme Regis: a distance of around 200 miles.
But among the difficulties this presented was obtaining enough barges to mount a successful crossing of the English Channel and getting enough troops massed all the way along the coastline.
There were also three more fundamental issues that needed to be addressed before an invasion could take place:
- the German air force needed superiority of the skies
- the German navy needed control of the English Channel
- the weather needed to be right
Achieving the first of these was considered to be of most importance. Without the total defeat of the RAF, no invasion of Britain was possible.
As such Hitler commanded the head of the German air force, Hermann Göring, to begin what became known as the Battle of Britain - in the words of Andrew Marr, "the single most important battle this country has fought".
Dan Cruickshank has written of how there is evidence that during the meeting of 31 July, Hitler had already decided that the invasion of England was merely a ruse concocted to intimidate Britain into surrender.
The German leader knew all along such an invasion was simply impractical, but wished to scare his foe into submission.
Yet "for the bluff to work, the build-up for invasion had to continue and Britain had to be kept under military pressure."
Whether this was true or not, Operation Sealion continued to be finalised.
Following disagreements between the German navy and army over the size of invasion, on 13 August Hitler agreed that the front should be narrowed and the size of German forces be reduced.
Operation Sealion now ran as follows:
One German force was to sail from Le Havre and land at Ventnor on the Isle of Wight, and also in the Brighton-Worthing area of Sussex, from where they were to establish a beachhead and break out towards Portsmouth.
Other forces were to sail from Boulogne to Eastbourne, Calais to Folkestone, and Dunkirk and Ostend to Ramsgate. Troops were to break out of their respective beachheads and head north to establish crossings over the River Medway.
Parachute troops were also to land near Dover and on the Downs above Brighton.
All forces would then join up to establish a front running from the Thames estuary to Portsmouth.
Reinforcements would be landed.
Troops in the Brighton area would then attack towards Basingstoke, Newbury and Oxford to secure crossing points over the Thames, before encircling London.
Finally the troops around the Thames estuary would march on London.
No troops were to march further north than the 52nd parallel (a line just south of Cambridge and Stratford-upon-Avon) in expectation that by this point the rest of the country would surrender.
This version of Operation Sealion was issued on 30 August 1940. Three days later, for the first time since the Battle of Britain began, British losses were heavier than those of Germany.
It looked very much as if the key factor that would allow Operation Sealion to take place - the destruction of the RAF - was imminent.
NEXT MONTH: How Hitler failed to invade Britain
Sources and further reading
The Storm of War, Andrew Roberts (2009)
The Making of Modern Britain, Andrew Marr (2009)
A History of Britain Vol. 3: 1776-2001, Simon Schama (2002)
The Origins of the Second World War, AJP Taylor (2001 reprint)
The German threat to Britain in World War Two, Dan Cruickshank
Wikipedia: Operation Sealion
History Learning Site: Operation Sealion
Defence Academy of the United Kingdom: Operation Sealion