By June 1940, Hitler’s Panzer Divisions had rolled up to the English Channel. The German Luftwaffe had just taken on the air arms of France and the United Kingdom, the two other most advanced air forces on the planet, and defeated them after a month-and-half of sustained aerial warfare.
The Royal Air Force was forced to hastily withdraw its fighter squadrons from France lest they were wiped out by attrition. Miraculously, 338,000 men of the British Expeditionary Force were evacuated at Dunkirk—leaving nearly all their heavy weapons (tanks, artillery, trucks) behind, as well as 68,000 soldiers killed or captured.
Germany’s Panzer Divisions could easily have smashed aside the U.K.’s battered defenders if only they could be ferried across the Channel—but therein lay the problem. How were the vehicles and men to be physically landed on a beachhead?
By that time, the Kriegsmarine had only tested a few prototype landing craft, and certainly did not possess the vast fleet of specialized, ramp-equipped vessels used by the Allies in landings in Normandy or Iwo Jima. The best solution the Kriegsmarine could come up with was to transfer barges known as Peniche and larger Kampine from the Rhine River to Calais and convert them. More than 2,400 were assembled for an invasion of the UK, codenamed Operation Sea Lion and modified with bow-ramps.
Of the barges, only 800 were self-powered, meaning the rest had to be towed at a speed of roughly two miles per hour. Some were later converted by the Luftwaffe with airplane engines, and others were loaded down with extra armor. Even the best of these riverine platforms were only minimally seaworthy, with top speeds of seven miles per hour. This is to say they were sitting ducks.
Germany also developed submersible modifications of its early-model Panzer III and IV medium tanks. The 25-ton tanks were fully sealed and used an 18-meter long hose attached to a buoy for an air supply. The tanks would roll off the ramps of seventy special Type B barges and sink to the seabed. The submarine-tanks would then crawl on the ocean floor to the beach. This method actually worked in tests—as long as the tanks didn’t bump into any obstacles or come to a stop for any reason, in which event they sank in the soft sand.
By August 1940, Germany converted more 252 Tauchpanzer III and IVs as well as fifty-two floating Panzer II Schwimmpanzer light tanks, organized in four battalions. Obviously never used in an invasion of the UK, a few instead saw action crossing the River Bug during Operation Barbarossa.
It’s fair to say Germany improvised in remarkably little time methods to land a large army on a nearby hostile shore. But those methods still relied on the other side having little ability to shoot back at the invasion force—and a little factor called the Royal Navy made that a very difficult issue to address.
Though before the war Hitler had an ambitious Plan Z to hulk out the Kriegsmarine with ten battleships and four aircraft carriers, that never came close to being realized. When the Kriesgmarine was heavily deployed in the invasion of Norway in April 1940, it achieved its objectives—at the cost of half of the German Navy’s twenty destroyers and three cruisers.
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The remaining handful of destroyers, cruisers and damaged battleships were in no position to defeat the Royal Navy’s Home Fleet. That was a task the Luftwaffe would have had to accomplish. However, sustained attacks by the bulk of the Luftwaffe only sank five British and French destroyers at Dunkirk (four more were lost to mines, motor-torpedo boats, and submarines), at the cost of 156 aircraft shot down by flak and RAF fighters.
Admittedly, the RAF lost a similar 145 warplanes providing air cover and ground support over Dunkirk. Therein lay the hint of a strategy—if the Royal Air Force could be bled out into ineffectiveness, then German air superiority might allow its bombers free reign to ravage the Home Fleet.
The Germans had 1,700 bombers and fighters to fling against 640 RAF Spitfires and Hurricanes, and the oft-forgotten Defiants (which didn’t have any forward firing guns!). The Germans could afford to trade losses. This was exactly what the Luftwaffe attempted to do through the first half of the Battle of Britain: first targeting British coastal convoys to lure the RAF into battles of attrition (the ‘ Kanalkampf’), then starting with the Adltertag (‘Eagle Day’) offensive on August 13, hammering radar sites and airfields in an attempt to destroy them on the ground. It was a strategy that steadily wore down the RAF’s Fighter Command limited pool of pilots.
However, after a British bombing raid over Berlin, Hitler ordered the Luftwaffe to retaliate by switching to nightly terror raids of London starting on September 3. The Blitz would kill thousands of civilians but give the RAF's hard-pressed fighter squadrons a badly needed reprieve, which they profited from to inflict unsustainable losses on the Luftwaffe. After sixty aircraft were shot down in one day of frenetic action over London on September 15 (now celebrated as Battle of Britain day), Hitler decided to throw in the towel, and Operation Sea Lion was canceled.
Did the RAF’s bomber pose a threat to an amphibious invasion? Certainly—but likely not a decisive one. Just as the Luftwaffe struggled to sink ships at Dunkirk and during the Kanalkampf, the RAF’s Coastal and Bomber command would have faced similar challenges. Early war British tactical bombers like the Blenheim and Battle were notoriously poor performers, unlike excellent successors such as the Mosquito and Beaufort bombers. Even then, during the infamous Channel Dash in February 1942, the RAF lost forty-two bombers attacking two German battleships, a heavy cruiser and six destroyers racing through the English Channel without sinking a single one.