After the defeat of France by Germany in 1940, Britain was Hitler's next target and documents which have just come to auction give an insight into how close the Nazis came to invading. Nigel Burton reports.

BRITAIN faced its darkest hour in 1940. Just ten months after the Second World War had begun France had fallen to the Nazis and the battered British stood alone.

But if Nazi generals believed this apparently hopeless situation would force the British to the negotiating table, they were wrong.

Faced with a choice of laying siege to the UK or launching a full-scale invasion, Hitler issued Directive Number 16 on July 16, which read: "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 carry out, an invasion of England..."

He placed the onus on General Franz Halder and announced the invasion force would be ready to sail by August 15. The operation was code-named Sealion.

Yesterday, the collection of maps and documents that formed Halder's invasion plans went under the hammer at an auction in Ludlow, Shropshire.

The collection reveals how Halder drew upon hundreds of coastal profiles, diagrams and photos of British towns as he assembled his scheme to occupy the islands using "overwhelming force".

As the deadline drew near, the Luftwaffe began to attack the channel ports and straits of Dover. Halder knew for any invasion to be successful, the Germans had to win air supremacy over the Channel.

Heavy guns were installed at Calais and used to bombard Dover 20 miles away. This forced the Royal Navy to withdraw its warships because of the threat posed by enemy aircraft - but it did not encourage the British to seek peace.

Despite their military superiority, the Germans still faced formidable obstacles. The invasion force had to cross 20 miles of open water, and had to land on a heavily-fortified stretch of coastline, a 200-mile stretch of the south coast from Ramsgate all the way to Lyme Regis, in Dover.

In order to give his generals more time, Hitler postponed the invasion until September 16, but the chances of success were fading.

Britain had mobilised her defences far more efficiently than the Germans expected.

The Local Defence Volunteers - later named the Home Guard and best known as the Dad's Army - had 1.5 million men ready to fight, while General Sir Edmund Ironside had devised a plan to slow the invasion and stall its forces on the beaches by fortifying South-East England with pillboxes, gun emplacements, trenches and minefields.

Ironside's "defensive crust" foresaw a war of attrition, forcing the Germans to fight for every step of conquered land all the way to Scotland.

Meanwhile, as the Battle of Britain ensured no aerial supremacy for Germany, Halder began to have second thoughts. At one meeting, he said Sealion was tantamount to "putting the troops through a sausage machine".

The plan was revised to narrow the invasion front and give two Panzer Divisions a better chance of breaking through the British defences.

It called for a front from the Thames estuary to Portsmouth. The force of only 60,000 infantry and 250 tanks would attack towards Basingstoke, Newbury and Oxford to secure crossings over the Thames and encircle London. As the problems became clear, even Hitler began to back away from the invasion. On September 17, Sealion was postponed indefinitely and the Germans turned their attentions to Russia instead.

In the auction, the battle plans sold for £450, while a biography of Adolf Hitler withdrawn from the sale was purchased privately for £1,250.

Richard Westwood-Brookes, of auctioneer Mullock Madeley, said: "Today, there are generations of people in this country who had no personal connections with those times, so it is easy to forget what might have happened if the RAF had not won the Battle of Britain."