What changed between the two world wars

Twenty-one years separate the armistice of 1918 from the invasion of Poland in 1939. That's less than a working lifetime. A great many of the men commanding armies in the second war had been junior officers in the first, and it isn't true that they learned nothing from it. They'd spent two decades arguing, in print and in staff colleges, about exactly what the first war had proved.

What changed between the two wars is mostly technical, and it comes down to the relationship between three things: how far you can shoot, how fast you can move, and how quickly you can tell anyone anything.

Get those three out of balance and you get 1916. Bring them back into balance and you get something that moves.

Why the first war stopped moving

By 1914 firepower had run well ahead of mobility, and the gap did the rest.

The magazine rifle, the quick-firing field gun and the machine gun had all matured in the previous thirty years. The French 75mm gun absorbed its own recoil hydraulically, so the barrel came back and returned to position while the carriage stayed put — meaning the crew didn't have to re-aim after every round and could fire fifteen times a minute or more. A water-cooled machine gun could keep going for as long as the belts and the water lasted. Barbed wire cost almost nothing and stopped men walking.

Against all of that, an attacking soldier moved at the pace of a man carrying sixty pounds across churned ground.

The killer was the asymmetry in reinforcement. If you punched a hole, your troops had to widen and exploit it on foot, over the ground your own artillery had just destroyed, dragging supplies through mud. The defender brought reserves up behind the line by rail and lorry, on intact roads. Every hour that passed favoured the side being attacked. That's arithmetic rather than stupidity, and it's what produced the pattern of offensives that gained a few kilometres and then died.

And the line had no end. Once the front stabilised it ran roughly 700 kilometres from the North Sea to the Swiss frontier. There wasn't a flank left to turn, which is what generals had been trained their whole careers to look for.

Nobody could talk to anybody

The single most underrated difference between the wars is the radio, and it's easiest to see by describing what commanders had in 1916 instead.

Telephone cable, buried if you had time and shells if you didn't. Runners on foot. Pigeons, which worked surprisingly well. Signal lamps, flags, coloured flares. Aircraft dropping message bags. Once an attack started, the wire got cut within minutes by the enemy's counter-barrage, and the men who went forward effectively disappeared from the map.

So an offensive had to be planned in full beforehand, timed to the minute, and couldn't be redirected. You can't change orders you can't deliver. If the left flank broke through and the right flank was pinned, nobody at headquarters knew for several hours, by which point the moment had gone. Commanders didn't refuse to adapt. They couldn't see.

Radio sets in 1918 existed but were heavy, fragile, and mostly used by aircraft and artillery observers. By 1939 a set fitted in a tank turret and a company commander could hear his battalion. That's the change that makes fast operations possible, and it's more important than armour thickness or gun calibre.

The first war did not end in stalemate

The trench image is so fixed that people assume the fighting stayed like that until Germany collapsed. It didn't.

Artillery quietly became a science between 1916 and 1918. Guns were individually calibrated for barrel wear. Meteorological data — wind, temperature, air pressure — was applied to firing tables. Accurate survey put every gun's true position on a common grid. Together these allowed predicted fire: opening a bombardment accurately without ranging shots first, which meant an attack no longer announced itself days in advance.

At the same time, sound ranging and flash spotting let gunners locate enemy batteries by triangulating the noise and muzzle flash, so counter-battery work became genuinely effective. The creeping barrage moved a curtain of shells forward at a set pace with the infantry following close behind.

Add tanks, ground-attack aircraft and better small-unit tactics and you get Cambrai in November 1917, and then the Hundred Days in 1918, which were mobile campaigns covering serious ground. The methods that everyone thinks were invented in 1939 were largely worked out in the last year of the first war, by both sides. What they didn't have was the engine and the radio to sustain an advance for more than a few days.

Machines that could keep going

A 1918 tank managed something like 6 km/h on good ground, broke down constantly, and cooked its crew. Its job was to crush wire and cross a trench alongside walking infantry. It wasn't meant to go anywhere on its own.

The interwar work fixed the mechanical problems rather than the concept. Better engines, proper suspension — the American engineer J. Walter Christie's design, with large road wheels on coil springs, allowed high speed across country and was picked up and modified abroad. A rotating turret became standard. Radios went in. Reliability improved enough that a formation could travel a hundred kilometres and still be a formation at the end of it.

Aircraft changed more dramatically than anything else. The typical 1918 fighter was a wood-and-fabric biplane doing about 200 km/h. Within twenty years the standard was a stressed-metal monoplane with retractable undercarriage, a variable-pitch propeller and a supercharged engine, doing well over 500 km/h. By 1944 there were jets flying operationally.

Bomb loads tell the same story more bluntly. A Gotha bomber of 1917 carried a few hundred kilograms. A four-engined heavy bomber of 1944 carried around 6,000 kilograms, much further. That's not an improvement, it's a change of category, and it's the technical basis for everything that happened to cities in the second war.

At sea, the gun line gave way to the aircraft carrier. Jutland in 1916 was the last great fleet action fought by battleships in sight of each other, and it settled remarkably little. The naval treaties of the 1920s capped battleship tonnage, which pushed several navies into converting large hulls into carriers because that tonnage wasn't restricted the same way. By 1942, fleets were fighting battles — the Coral Sea is the usual example — in which the surface ships never saw one another at all.

Information became a weapon in its own right

Two systems matter here, and neither is a gadget. Both are organisations.

Radar was being developed in several countries during the 1930s. Britain's coastal chain was operating before the war began, but the radar sets aren't the interesting part. What mattered was the reporting network built behind them: filter rooms that reconciled contradictory plots, an observer corps tracking aircraft once they crossed the coast and the radar could no longer see them, dedicated telephone lines, and a command structure that could put fighters in the right patch of sky at the right time. Without that plumbing, radar returns don't tell you anything. Standing patrols burn out squadrons; being told where to go doesn't.

Codebreaking went the same way, from a room to a factory.

In the first war Britain's Room 40 read German naval traffic and, most famously, decrypted the Zimmermann Telegram in 1917. Clever, small-scale, and dependent on captured codebooks. By the second war the problem was machine ciphers generating an astronomical number of settings, and the answer had to be mechanical too.

The Polish Cipher Bureau — Marian Rejewski, Jerzy Różycki and Henryk Zygalski — reconstructed the wiring of the German Enigma machine mathematically during the 1930s and built devices to help find daily settings. In July 1939 they passed their work to British and French colleagues. At Bletchley Park the effort scaled up enormously: Alan Turing and Gordon Welchman's bombe searched settings electromechanically, and from 1943 the Colossus machines attacked a different, higher-level German cipher electronically. Thousands of people worked on the traffic, most of them women, most of them doing careful repetitive work rather than flashes of genius.

The result wasn't magic. It was a steady flow of intelligence that made convoy routing, air interception and operational planning much better bets than they'd otherwise have been.

Logistics, and the horse that nobody mentions

Popular memory has the first war as horses and the second as engines. It's muddier than that, and more interesting.

Both wars ran on railways up to a railhead and then struggled. Beyond the railhead, first-war armies used horses and mules for almost everything, and the animal casualties were enormous. But the second war didn't sweep that away. The German army that invaded the Soviet Union in 1941 still moved the bulk of its artillery and supply with several hundred thousand horses, with fully motorised formations forming a minority spearhead. Only a handful of armies were genuinely motorised throughout.

What did change was industrial output and standardisation. Mass-produced trucks and light utility vehicles, purpose-built landing craft, and merchant ships built to a single design in weeks rather than months. Fuel became the binding constraint on operations in a way it had never been in 1916, when an army that ran out of oats simply requisitioned more.

The infantryman, meanwhile, still walked. In both wars, most soldiers moved on their feet and carried their kit, and most of them were killed or wounded by artillery rather than by anything they could see.

Medicine caught up, then overtook

Here's a shift with numbers behind it, and it starts in the first war rather than the second.

Wounds in 1914 were usually contaminated with heavily manured farm soil, which made gas gangrene and tetanus common. The response was surgical rather than pharmaceutical: cutting away dead tissue promptly, leaving wounds open, irrigating with antiseptic solution. Blood transfusion became practical during the war once sodium citrate was used to stop clotting and refrigerated blood could be stored and carried forward. The Thomas splint, applied at the front for fractured thighs, cut mortality from that injury dramatically over the course of the war.

Between the wars came the sulphonamides, the first drugs that reliably killed bacteria inside the body. Then penicillin, discovered in 1928 but useless as a treatment until Howard Florey's Oxford team showed it worked and American manufacturers found a way to grow it in deep fermentation tanks. From 1943 it was available in quantity.

Alongside that: blood plasma that could be dried and reconstituted, better forward surgical units, systematic triage, insecticides that suppressed the lice carrying typhus, and antimalarials produced synthetically. The old pattern where armies lost more men to disease than to fighting had already begun to break in the first war and broke properly in the second.

Who was killed changed

It's the hardest part of the subject to write plainly. The plain version is the right one.

The deaths of the first war fell overwhelmingly on soldiers. Civilians died in large numbers, but mostly indirectly — through blockade, food shortage, displacement and disease, and then through the influenza pandemic that swept a weakened world in 1918 and 1919. Total deaths are usually put somewhere in the range of 15 to 20 million, with wide uncertainty.

In the second war the majority of those killed were civilians. Estimates for total deaths generally fall between 60 and 80 million, and the range is that wide because record-keeping collapsed in several of the worst-affected regions and because historians count famine and occupation deaths differently.

The mechanisms behind the shift are identifiable. Aircraft with the range and payload to reach cities, and doctrines that treated industrial and urban targets as legitimate. Sieges and blockades imposed on populations rather than armies. Occupation policies that deliberately took food. Forced movement of people on an enormous scale. And systematic mass murder carried out as state policy, most of all the Holocaust, in which around six million Jewish people were killed.

Those are facts about how the second war was fought. They aren't a reflection on any nationality, and the record doesn't sort neatly by flag.

What hadn't changed at all

Set the technology aside for a moment and a lot of the second war would have been recognisable to someone from the first.

• Artillery remained the biggest single cause of battle casualties in both wars, by a wide margin.
• Most infantry still carried bolt-action rifles, not automatic weapons.
• Digging in still worked. Wherever the front stabilised, trenches came back immediately.
• Weather, mud and terrain still governed what was possible, and rain still stopped offensives.
• Feeding, clothing and moving millions of people remained a harder problem than killing them.

The psychological injuries didn't change either, though the words did. What was called shell shock in one war was called combat fatigue in the next, and it's the same condition, understood slightly better and still badly treated.

The last change

In August 1945 two nuclear weapons were used on Japanese cities, and the terms of the whole subject changed.

Everything else described here is a matter of degree — more range, more speed, better information, faster surgery. Between 1918 and 1939 the tools got better at doing recognisably similar jobs. A single aircraft destroying a city isn't a matter of degree.

The wars themselves make more sense as engineering and organisation than as narrative. Firepower outran movement, and the front froze. Then engines, radios and radar let movement catch up, and the front unfroze — and because aircraft could now reach anywhere, the front stopped being a line at all.