The British Army had little chance of stopping the Germans if they had managed to get a large force ashore in June or July 1940.
However, as in previous invasion threats the main defense would devolve onto the Royal Navy and now the RAF. Britain’s Fighter Command had lost many aircraft and pilots in the Battle of France and could muster only 331 Supermarine Spitfires and Hawker Hurricanes. But German indecision and a need to redeploy and refit the Luftwaffe gave Britain a decisive respite.
By September Britain had built up its armored forces to nearly 350 medium and cruiser tanks. Coast defenses were much improved. Strong reinforcements had arrived from Canada. However, General Sir Alan Brooke, Commander in Chief (C-in-C) Home Forces, on September 13 confided pessimistically to his diary that of his 22 divisions “only about half can be looked upon as in any way fit for any form of mobile operations.”
On August 11, the eve of Eagle Day when the Luftwaffe was to launch its offensive to gain air superiority over southern England, RAF fighter command had 620 Spitfires and Hurricanes and aircraft production was exceeding totals called for.
The Defense of Britain: Air, Land, and Sea
For the Royal Navy the advent of airpower posed several problems. No longer could the Navy alone deny the sea to an invader as in 1588 when Catholic Spain attempted to invade England by sea and in 1804 and 1805 when Napoleonic France attempted the same thing. The Royal Navy hoped by the use of bombardment and mines to attack the invasion fleet before it even left its ports. If such attacks were not decisive, it would attack the invasion flotillas as it arrived off the English coast. The Luftwaffe would be stretched to the limit.
As the invasion beaches were not known, the Royal Navy needed to cover an area from the Wash to Newhaven. It had the strength to carry out that mission. The British Admiralty contemplated “the happy possibility that our reconnaissance might enable us to intercept the expedition on passage.” Given the speed of the invasion barges, taking 12 hours to cross the Channel was a near certainty. The main forces to be used were destroyers and light craft, with close support of cruisers. It was agreed that the battleships should only come south if German invasion transports were escorted by heavier German ships.
Admiral Sir Charles Forbes, C-in-C for the British Home Fleet, argued that so many ships should not be pulled away from the very real German threat to the convoy routes. Forbes kept his battleships, but many of his cruisers and destroyers were dispersed in ports around the southern and eastern coasts. Forbes was proved right with so many ships committed to static roles. Losses among the convoys began to mount.
The RAF also had a vital role in defeating an invasion. Bomber Command would attack shipping as soon as it began to assemble. Once the invasion began, Fighter Command would move to the offensive against troop-carrying aircraft and supply air cover to the Royal Navy attacks on enemy shipping. Coastal Command would support the Navy as well and join Bomber Command in attacking shipping.
Gradually the emphasis and reserves shifted toward southeastern England. Here the sea crossing was the shortest and the beaches would be within German fighter protection. On September 4, a memo warned that if the Germans “could get possession of the Dover defile and capture its gun defenses from us, then, holding these points on both sides of the straights they would be in a position largely to deny those waters to our naval forces.” With this warning the chiefs of staff moved more ground troops into this vital sector.
The End of Operation Sea Lion: Hitler’s Attention Turns East
On September 7, intelligence warned that a German invasion was near. Tide and light conditions would favor the enemy between September 8 and 10. The Royal Navy put all its small craft and cruisers on immediate notice and stopped all boiler cleaning. The RAF moved from Alert 2 invasion in three days, to Alert 1 invasion imminent within 12 hours. It was decided to issue the code word “Cromwell” as a warning to take up battle stations. Unfortunately, a lot of recipients did not know its meaning. Some Home Guard units assumed it meant the invasion had started and rang church bells, which was an agreed warning of invasion, and blocked roads.
The chiefs of staff met in London under Prime Minister Winston Churchill’s chairmanship on September 7, while London was subjected to a massive air raid.
Soon, however, the crisis began to wane. The Germans, stung by an RAF raid on Berlin, switched their attack from RAF fighter bases to London allowing the RAF to make good its losses. The prize of air superiority rapidly slipped away. On September 14, Hitler postponed the invasion until September 17 due to Luftwaffe losses. Then, on September 17, it was postponed again. On September 20, the Germans began to disperse the invasion barges, of which some 10 percent had already been sunk or damaged by the RAF and the Royal Navy.
For the British, the invasion threat remained well into October, but Hitler’s mind was no longer on England, if it had ever really been firmly fixed in that direction. Rather, it was turned east toward Russia.
In the 1970s, the Department of War Studies at the Royal Military Academy of Sandhurst made Sea Lion into a war game based on the plans of both sides. A panel of generals, admirals, and air marshals umpired the war game. Any disputes over exact losses were settled by cutting cards. Admiralty weather records were made available, which proved the situation would have been favorable to an invasion between September 19 and September 30. Such findings validate Sea Lion as one of the great “what ifs” of modern military history.
This article by Mark Simmons originally appeared on Warfare History Network.
Image: Wikimedia Commons