A 1974 wargame hosted by Sandhurst Military Academy suggested that a Nazi invasion of the United Kingdom in the fall of 1940 would have been a disaster for Germany.

One of modern history’s greatest “what ifs” is whether Nazi Germany could have successfully invaded the island of Great Britain in 1940.

The summer of 1940 was a dark time in world history. After the fall of France, Nazi armies had conquered Europe, holding sway from the coasts of Normandy to the new German-Soviet border in what had once been Poland. At that time, Germany and the Soviet Union were allies; the Nazis’ betrayal would not take place for a full year. And while the United States was clearly sympathetic to the Allied cause, isolationist sentiment remained strong, preventing it from entering the war. The last holdout against Hitler was the United Kingdom, which newly-inaugurated prime minister Winston Churchill declared would “never surrender.”

To end the war, Germany proposed an amphibious invasion of southern England, known as Operation Sea Lion, in the fall of 1940. In order to make the invasion work, the Nazi high command agreed that it would need to suppress British control of the air. Accordingly, the Luftwaffe sent waves of aircraft to attack RAF positions on the island, leading to the months-long Battle of Britain. After the RAF fended off those attacks, the Nazis determined that Operation Sea Lion was unworkable, and it was canceled; Germany never attempted to cross the English Channel, and the two sides remained fortified on their respective coasts until the D-Day landings on June 6, 1944.

What if the Nazis had pressed forward with the invasion, despite the lack of air cover? Three decades after the war, in 1974, Britain’s Royal Military Academy Sandhurst attempted to discover the answer—recreating the invasion as a detailed military war-game using historical orders of battle, logistics, weather, and command decisions. The result? Germany secured early successes, but the invasion ultimately failed. 

The world will (perhaps thankfully) never know what really would have happened had Hitler invaded Britain in 1940. But it is fascinating to consider how it might have played out. 

Amphibious Invasions Are Really, Really Hard

Hitler had planned for Operation Sea Lion to commence in the fall of 1940, following a Nazi victory during the Battle of Britain. The objective was to land German forces across the English Channel, establish beachheads, capture British ports, send over more troops, and ultimately march on London. The planned invasion force called for roughly 90,000 first-wave troops, with follow-on reinforcements after securing ports. Success depended on a multitude of conditions: a meaningful Luftwaffe victory over the RAF, the powerful Royal Navy remaining outside the Channel, and favorable weather for a seaborne invasion. These were all giant ifs.

Amphibious invasions are among the hardest military operations, and have been since the beginning of time. No European army had landed on British shores and won a victory since 1688. Hitler’s position in Europe in 1940 was paralleled by Napoleon’s in the early 1800s; though the French dictator effectively ruled over all of continental Europe, he did not have the strength to mount a naval invasion across the channel, and he knew it.

The D-Day invasion, which took place four years later, gives some perspective into the challenges that Operation Sea Lion would have faced; the Allies spent years preparing for it, incorporating lessons learned from a failed raid against the port of Dieppe in August 1942. At the time of the Normandy landings, the Allies had the benefits of overwhelming naval and air superiority and specialized landing craft. Germany had none of these advantages. Instead, it would have relied on converted Rhine river barges—which were slow, unstable, not designed for rapid unloading of troops, and poorly suited for the rough waters of the Channel.

Success for Hitler was deeply dependent on the Battle of Britain. Sea Lion could not proceed without air superiority. And despite suffering heavy losses, RAF Fighter Command was able to remain intact. Britain also enjoyed an integrated air defense system, including Chain Home radar, the Observer Corps, and centralized fighter control. The Luftwaffe, despite their efforts, was unable to destroy the RAF or Britain’s air defense system, and eventually shifted towards bombing London in a futile attempt to turn British morale against the war. The “Blitz” bombing campaign lasted from September 1940 to May 1941, but neither broke British morale nor did much strategic damage. By the end of the bombing campaign, it was clear to both sides that the UK was in the war to stay—and within months, it would find another ally after Hitler made the fateful decision to turn on his erstwhile partners in Moscow.

The Sandhurst Exercise Suggests the Nazis Made the Right Decision

But the “what if” remained—and in 1974, the Sandhurst war-game sought to answer that lingering question. Conducted using both British and German senior military officers and historians, including many participants in the original Battle of Britain, Hitler’s plan was recreated as closely as possible. Participants were constrained by what each side actually knew in 1940 and were derived the benefit of hindsight or modern weapons. Logistics, tides, weather, naval movements, and available aircraft were all considered with historical accuracy. 

What did the exercises find? After establishing several initial footholds, the Germans achieved limited tactical success through airborne and amphibious landings. But problems emerged almost immediately, including poor weather, shipping losses, difficulty reinforcing beaches, and mounting supply shortages.

The initial success did not translate into a sustained offensive operation. The Royal Navy, vastly superior in all respects to Germany’s Kriegsmarine, was a crucial thumb on the scale in favor of the UK; its destroyers and cruisers would have entered the Channel to confront invasion vessels once the attack began, which Germany would not have been able to suppress through airpower. Accordingly, Germany barges carrying reinforcements would have been highly vulnerable to naval gunfire.

Germany’s reinforcements would have collapsed and the invasion force would have become isolated. Once isolated, fuel, ammunition, food, and replacements would have been cut off. The beachheads would have become unsustainable. And ultimately, German forces would have been destroyed, or forced to surrender—and Hitler would have been dealt a grievous blow, likely altering his strategic decision-making in the years that followed.

At least that’s what the British wargame concluded. Fortunately, we’ll never know what would have actually happened.

About the Author: Harrison Kass

Harrison Kass is a writer and attorney focused on national security, technology, and political culture. His work has appeared in Tablet, City Journal, The Hill, The Spectator, and The Cipher Brief. He holds a JD from the University of Oregon and a master’s in Global & Joint Program Studies from NYU. More at harrisonkass.com.

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