Britain is an island, and an island is a wonderful thing to defend and a terrible thing to feed.
In nineteen thirty nine, Britain imported around two thirds of its food. It imported all of its oil, all of its rubber, most of its iron ore, and the aluminium its aircraft were made from. Every single one of those things arrived on a ship.
Which meant that Germany did not need to invade Britain at all. It only needed to sink enough ships, for long enough.
Churchill wrote afterwards that the only thing that ever really frightened him during the war was the U-boat peril. Not the threat of invasion. Not the Blitz. A line on a graph, drawn every week, showing how much shipping had gone to the bottom.
The obvious way to protect merchant ships is to spread them out, so that a submarine looking for one has a great deal of empty ocean to search.
The obvious way is wrong, and working out why is one of the most useful pieces of thinking in the whole war.
Group forty ships together into a convoy and you have not made a bigger target in the way it feels like you have. The ocean is so vast that a submarine's chance of finding any given convoy is barely different from its chance of finding any given single ship. A convoy is only a few miles across, and the Atlantic is three thousand miles wide.
So by grouping the ships, you have reduced the number of things to be found from forty to one, without meaningfully increasing the chance of finding it.
And when the submarine does find your convoy, it cannot sink forty ships. It carries perhaps fourteen torpedoes, it has to reload, and it is now surrounded by escorts who know exactly where it is.
Meanwhile, the escorts you have are all in one place instead of scattered. Six destroyers protecting forty ships is a real defence. Six destroyers protecting forty ships spread across the ocean is nothing at all.
A convoy was a rectangle of ships, wide and shallow, typically nine or ten columns across and four or five ships deep, with about a thousand yards between each. From the bridge of a ship in the middle you could see nothing but other ships.
It moved at the speed of its slowest member, which was often about seven knots. That is roughly the speed of a bicycle being ridden by somebody who is not in a hurry. At that speed the crossing from Halifax to Liverpool took the better part of a fortnight.
The ships showed no lights and kept radio silence, and they had to hold formation in fog and gales, in the dark, without lights, for two weeks. Collisions were a constant danger and a regular occurrence.
There was also the question of what happened if you were hit. In the North Atlantic in winter, a man in the water has minutes. And a convoy under attack could not stop to pick people up, because a stopped ship is an easy target and stopping puts everybody else at risk.
So specific ships were designated as rescue ships, at the back, whose job was to fall behind and pick up survivors while everybody else kept going. It is hard to think of a worse job at sea.
The German answer was to stop hunting alone.
A single submarine that found a convoy would not attack. It would shadow it, out of sight over the horizon, and radio its position back to headquarters in France. Headquarters would then order every other submarine within reach to converge, and only when a group had gathered would they attack, at night, on the surface.
On the surface is the part that surprises people. A submarine of that era spent most of its life on top of the water, because it was much faster there and its batteries only lasted a day underwater. At night, low in the water and unlit, it was very nearly invisible, and it could keep pace with a convoy.
The tactic was called the wolf pack, and in the middle years of the war it was devastating. There were convoys that lost a third of their ships in a single crossing.
But notice the weakness built into it. The whole method depended on radio. The shadowing boat had to transmit, and headquarters had to transmit back, over and over, every night. Which meant that every attack began with the enemy telling you where he was.
Aircraft turned out to be the answer, and not because they sank many submarines. They mostly did not.
What an aircraft did was force a submarine to dive. A submerged submarine is slow and half blind and cannot keep up with a convoy, so an aircraft simply appearing overhead broke off the attack before it started.
Convoys therefore had air cover for the first part of the crossing, flying from Britain or Iceland, and for the last part, flying from Canada. And in the middle there was a stretch of ocean, several hundred miles across, that no land-based aircraft could reach and return from.
It was called the Mid-Atlantic Gap, and the crews had a blunter name for it. They called it the Black Pit. Every submarine commander knew where it was, and that is where the convoys were attacked.
Closing it took two things, and both took far too long. The first was a small number of very long range aircraft, converted heavy bombers with extra fuel tanks, which could just reach the middle and stay a little while. The argument over whether those aircraft should go to the Atlantic or to the bombing campaign over Germany was fought bitterly for two years, and the Atlantic kept losing it.
The second was to bring the airfield along, in the form of small escort carriers built on merchant hulls. Once a convoy carried its own aircraft, there was no gap anywhere.
In March nineteen forty three the war in the Atlantic was arguably being lost. Losses that month were appalling, and there were serious internal discussions about whether the convoy system itself had failed.
Two months later it was over.
In May nineteen forty three, forty one German submarines were sunk. On the twenty fourth of May, the admiral commanding them withdrew his boats from the North Atlantic. He lost one son in a submarine that month, and the other a year later on a fast attack boat. His own crews called that month Black May.
Nothing single caused the reversal. The gap had closed. Escort groups had become experienced and were now organised into hunting teams that could stay with a contact for a day if necessary. A short-wave direction finding set had appeared that could take a bearing on a submarine's radio transmission from the deck of an escort, which meant the shadower's own message gave him away. Ship-borne radar had improved to the point where a surfaced submarine at night was no longer invisible. And the codebreakers, whose story is the next few episodes, were reading the orders that sent the wolf packs out.
Every one of those was a small advantage. Together they turned the hunter into the hunted inside eight weeks.
The Battle of the Atlantic ran from the first days of the war to the last, longer than any other campaign of the war, and it never stopped for a single week.
The people who paid for it were mostly not in any navy. Merchant seamen were civilians. They were not soldiers, they had not been conscripted into a fighting service, and for much of the war a merchant sailor's pay stopped on the day his ship sank, which is a fact that caused enormous and entirely justified anger at the time. Around thirty thousand British merchant seamen were killed, a higher proportion than in most branches of the armed forces.
The other side of it was terrible too. Germany sent about forty thousand men into submarines and roughly three quarters of them did not come back, which is close to the worst survival rate of any fighting service on any side in the war.
Here is something to think about tonight. The convoy answer was not obvious. It felt like putting all your eggs in one basket, and everybody's instinct said to spread out. It took arithmetic to see that the instinct was wrong. When has an answer felt wrong to you and turned out to be right anyway?