Improved logistics.

The irony of modern warfare, as found by Trevor Dupuy, is that daily loss rates in battle have decreased as the lethality of weaponry has increased. While bloody disasters like the first day on the Somme loom large in the minds of our collective memory of the war, the reality of combat during WWI was that it was comparatively far less dangerous, on a day by day basis, than fighting during the Napoleonic wars or the American civil war. Things like gas, while visually horrific, made a fairly small dent in casualty numbers, as they were mostly a suppressant tool, not a killer. While machine guns played a decent sized role in producing casualties, they were still, as with gas, mostly a tool to suppress offensives while artillery, the real killer, did its deadly work.

Rather than improved weapons, made the war so deadly was the logistical ability to sustain that fighting for much longer. The Western Front, in particular, was fought in the area between the two largest rail corridors in the world, with the German line being supported by the 4 track line that went through Sedan. While four tracks may not seem like much, when it comes to railways it's a bit like having a hundred lane highway, you can sustain a colossal effort through that kind of transport artery. In addition, beyond the ability to just transport goods and men to the front, the first and second industrial revolutions (in particular the chemical aspect of the 2nd industrial revolution) meant that nations were also capable of producing the goods necessary to carry on the war in much greater quantities. In particular, the sheer quantities of ammunition produced to fuel the fighting are utterly staggering to take into account. The British produced 550,000 tons of 18 pounder shells during 1917 alone, and that's just one year's production for one weapon system (even though it was a key one). Finally, you also have to consider the massive expansion of the capabilities of the administrative state during the late 1800's. The governments of Europe were capable of tapping and organizing a much large portion of their manpower and resources than they were during periods like the Napoleonic wars, further increasing the capacity to keep the fight going.

So what does this mean? In earlier times armies could horrifically bloody each other on the field, but they could only sustain it for 1-2 days before they had to leave the field to rest, replace losses, and rearm. By contrast, the massively expanded logistical capacities that armies had access to during WWI meant that they could simply bring up more men and materiel and make another go at it the next day, and the day after that, and the day after that. Armies didn't need to leave a position because they could be reinforced faster than the enemy could progress, and, after 1914, fighting was never forced to peter out because of a lack of munitions.

This was hardly the first time this had occurred either. While comparing the first world war to the American civil war is, in my view, a poor analogy and badly played out, it is interesting to compare the logistical capability of the Army of the Potomac and Army of Northern Virginia to just keep bashing into each other for nearly a month during the Overland campaign to the need for Napoleonic armies to separate from each other. What separates WWI is that the Overland campaign brought both the Union and Confederates nearly to the breaking point (Union public sentiment towards the war was at an absolute nadir, and Grant was stripping the Washington defenses of what men they had left, leading to the Army of the Potomac's terrible performance in the early stages of the Siege of Petersburg, while the Confederates burned through their last reserves of willing, trained, manpower) while the states of Europe during WWI had access to a much larger resource and manpower pool, as well as better organizational tools.

It's why trying to define clashes during WWI as "battles" doesn't necessarily work. The "battle" of Verdun, for example, lasts for nearly a year and encompasses nearly 10 distinct major offensives and dozens upon dozens of minor discrete engagements. Rather, what we are talking about is a campaign where men and materiel are constantly being shuttled in as the armies take small pauses before constantly reengaging. Even the Marne, which occupies a small time frame with decisive battlefield actions, is really a massive collection of individual engagements ranging across a frontline more than 120 miles long. The ability to operate on such a grand scale, and to sustain those operations for a long time, is what created the massive butcher's bill of WWI.

In short, for the soldier on the ground, he was safer, even during major offensives, on a day by day basis, in the trenches of the Western Front than in Napoleon's army whenever it faced an adversary. However, what racked up the massive death toll is that his army was capable of sending him into battle for far more continuous days, was able to keep far more soldiers like him in the field and in combat than before, and was able to find, conscript, train, arm, and then send in his replacement if he died with far greater speed and efficiency than before. That's how you get campaigns like the Somme where more than a million people become casualties.