How did the Allies win WW1? So this is a pretty big ask as there’s a lot of moving parts in the Allied war effort but I think I can break it down into three (because historians love threes) broad areas. We’ll call them Blood, Steel and Brains.

Blood – Is more or less the numbers argument. From the outset the Central Powers were outnumbered by the Allies due to the immense population of Russia and the British Empire. Once the US joined the war the numbers went even further into the Allies favour. This enormous pool of manpower dwarfed that of the Central Powers and meant that unless a decisive victory was won quickly on the battlefield the Allies would most likely win a long war.

Steel – By this I’m really talking about the Allied industrial and economic power. Again due to large empires and then the entry of the US, the Allies were able to draw on vast reserves of resources and industrial capacity to fight the war. Especially by the vital year of 1918, with the naval blockade biting Germany, the Allies were at their highest level of output. Ammunition, guns, tanks, food and all the other supplies needed to fight were being turned out at a greater rate than the Germans could compete with. When German troops captured British and French positions in the Spring of 1918 they were dismayed to see how well the Allied troops were eating and when compared to themselves.

Brains – And by brains, I mean all of the technical, tactical, doctrinal and strategic advances made by the Allied armies in four years of fighting. While we may think of WW1 as only trench warfare, the reality is that the fighting went through a transformation during the war, and it wasn’t only the Germans who picked up new ideas. The Allies were able to outfight the German armies on the battlefield.

Blood and steel I’ll combine for brevity and cause I think numbers are a little boring. Based on populations alone it’s quite easy to see that the Allies had a huge advantage over the Central Powers. Taken all together the Allies had a total population base of ~700 million against the Central Powers ~200 million. Now a state’s ability to actually raise armed forces from these populations varies a lot, but the main players in the Allies are still able to raise enough troops to outnumber the Central Powers. Along with manpower the empires of France and Britain provided enormous reserves of vital war resources like coal, oil, steel, rubber, lead as well as vital foodstuffs. Also available to the Allies was the immense industrial base of Britain and America. Britain was in 1914 still the workshop of the world, but the US and Canada were also rising industrial powers. Although it took some time to convert civilian industry to war footing the end state was a mammoth military industry that powered not just the British War effort but also considerably aided that of France, Russia and the US.

German industry simply couldn’t keep pace with the spiralling cost of fighting the war. Ever more ammunition was needed for the guns and the siege like nature of the fighting meant that it was extremely costly in terms of materiel. Historian Meleah Hampton has noted how in 1918 as the Allies were turning increasingly to a materiel centred approach to fighting the Germans had to rely on a more human centred approach as their supply situation became precarious.

Brains is, I think, the most important area, but I’m probably biased. As they fought the war the Allies underwent what has previously been called the “learning curve” but is now more accurately called the “learning process”. The ideas of how war would be fought leading up to 1914 turned out in many ways to be faulty, but these problems were compounded by a variety of novel problems that hadn’t been previously considered. Issues like deep belts of barbed wire, complex trench networks extending miles back, the inability to neutralise enemy artillery, gas, tanks, light mortars and a score of other things made the problem of fighting, on the Western Front at least, a very difficult challenge. And often every solution raised new, different problems to be solved.

Over several years of fighting the Allied armies tackled these problems, tried solutions, discarded failed options and eventually came up with a winning concept. It was realised that the war was attiritonal and that an outcome from a decisive battle was unlikely. While the Germans could, and did, pull back to high ground and fight a defensive war in the West, coming up with ingenious methods of defence in depth, the Allies were forced to fight offensively. The Germans were occupying French and Belgian territory and had to be removed. This meant offensive operations. And in 1915, 1916 and 1917 the Allies repeatedly launched offensives to dislodge or destroy German troops. Strategically they usually failed, but there were often many operational or tactical successes. And even the failures were useful if lessons could be gleaned from them. And although the progress was uneven, the Allies did move forward, both literally and metaphorically.

But I think the thing that really did it for the Allies was Foch being appointed Supreme Commander on the Western Front. It was at this point when the Allies were able to come up with a strategic and operational concept that would bring them victory. After the failures of the German Spring and Summer offensives it was the Allies turn to strike. Starting with the French at Soissons and then switching to the British at Amiens they were able to launch consecutive, rapid (for the time), successful limited offensives against German positions.

Making use of secrecy to keep the Germans off balance about where they were going to strike and then using devastating artillery barrages and often supported by tanks the French and British were able to break into German vital positions, taking trenches, men and guns and forcing neighbouring positions to fall back. The Allies then advanced again until German reserves were rushed in and resistance stiffened and they shut their offensive down. As soon as one offensive stopped another army would start up again, pushing the Germans off balance and forcing them to rush reserves to the newly threatened part of the line. These hammer blows pushed the Germans back in what is known as the Hundred Days Offensive, which ended with the Armistice on 11 November.

There’s more detail and a lot of other things going on all the war through this, but I think it’s really those three areas that determine the outcome of the war.

Whilst you await a more specific answer the faq may help