"If we can’t capture a port, we must take one with us.”

Both the Allies and the German army understood that the key to the Battle of France was logistics. It did not matter how many men Britain and the US landed on the beach; to defeat the Germans they had to land tanks, heavy equipment, and an unfathomable quantity of ammunition, fuel, and other necessities. To accomplish that, the Allies would need ports and harbors.

After the raid on Dieppe in 1942, the allies also recognized a direct assault on a port was almost certainly impossible. Edward Ellsberg from The Far Shore:

The German General Staff, a body governed in its military thinking solely by logic, had early figured the problem out to its one logical conclusion—cold logic showed a successful invasion to be impossible. Their advice to Hitler consequently had been, “Hold the ports and we hold everything.” And thus ran their reasoning (which no one, whether on the German side or on ours, could refute): A large, mechanized army, such as von Rundstedt and Rommel had, covering the Atlantic Coast from Denmark to Spain, could be defeated (if at all) only by a larger, better mechanized army—an invading army of a million men, at least, formidably equipped. Conceded that the Allies might, with their superior sea power, somehow land somewhere on the open European coast the larger army needed, they still could not land the heavy tanks, the big guns, the mechanized equipment and continuously disembark the immense quantity of supplies required to make that army an effective fighting force, without the wharfs, the harbor cranes, and the huge protected harbors necessary in all kinds of weather to handle ashore heavy equipment and supplies in such vast quantity.

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The only possible conclusion? An invasion, yes, if the Allies are so mad as to be willing to offer up a million ill-equipped men to be massacred by Field Marshal Rommel’s mechanized forces. But a successful invasion? Obviously an impossibility! To that conclusion, the German General Staff, the British War Office, the American strategists, including Eisenhower’s Chief of Staff, General Walter Bedell Smith, ultimately all subscribed without dissent.

And this is why GB Shaw was correct: "All progress depends on the unreasonable man."

But the British are a most illogical and stubborn race. Had they been more logical and less stubborn, they would swiftly have surrendered to Hitler after the Fall of France, and the question later of how successfully to stage an invasion impossible of success would never have risen to plague them. But running true to British doggedness even in the face of inevitable defeat, they neither accepted defeat after Dunkirk nor the impossibility of landing once again in Europe, even after their disastrous attempt at Dieppe. Doggedly the British planners continued to butt their heads against the stone wall of that impossibility. They continued to get nothing for their efforts except more headaches.

And then the pay-off:

The embattled planners, stymied, could only glare ferociously at each other across the conference table, blood-pressures rising dangerously. At this juncture, when it seemed most likely that British officers and gentlemen were about to forget that they were either, Commodore John Hughes-Hallet, senior Royal Navy planner, rose, stood a moment rolling his pencil briskly between his palms, then with mock solemnity tossed in his solution for the impasse.

“Well, gentlemen, all I can say is this—if we can’t capture a port, we must take one with us.”

All hands—soldiers, sailors, airmen alike—roared heartily at this merry conceit—fancy that, a whole seaport afloat, being towed across the Channel. A good joke, Commodore, worthy of more wine! They had it. Tensions relaxed. With everyone still laughing, the meeting broke up, with any solution to the port problem no nearer than before.

But by morning, the uproarious jest of the night before had begun to haunt both the jester himself and the most important of his hearers—Lieut. General Sir Frederick Morgan, Chief of the Planning Staff. That silly idea—floating a seaport across the Channel—was the only alternative. Silly then or not, might not that sole alternative, taken seriously somehow be made a reality? Morgan and Hughes-Hallett, looking hopefully at each other next morning, agreed that possibly it might. Hughes-Hallett was assigned to develop it. And so in June of 1943 was conceived what was to become Operation Mulberry.

The Mulberry harbors performed wonders. When combined with Allied air power, which strangled German resupply efforts, they gave the West the crucial edge in the build-up which set the stage for victory in the West.