The conventional explanation for why the Western Front in World War I settled into a stalemate is that the power of defensive weapons was stronger than the offensive methods employed. The theory is that the defensive potential of machine-guns, artillery, repeating rifles, and trenches was unbreakable with infantry and artillery alone. This simplistic explanation does not suffice under close scrutiny though. If this were so, why were the Germans not stopped in France until after they had removed troops to the Eastern front for the Battle of Tannenberg and why were the French stopped cold when they attempted to invade Germany in August 1914?
The reasons for stalemate are complex; they are both systemic and technical. The machine-gun certainly played a major role but even more decisive was the cult of the offensive prevalent in the British and especially French armies. The French disdained anything but the headlong attack, they thought the way to fight was to charge with the bayonet and trained their army that way. This accounts for the staggering number of casualties the French army suffered in the opening months of the war, over 350,000 French soldiers died by the end of August 1914. The British army was the only completely regular force in Europe and their performance in the opening months showed it. They stopped the Germans at Mons for two days using their rifles. They were trained to fire fifteen aimed rounds per minute and the Germans thought they were using machine guns when in reality they faced rifle fire.
Among the technical reasons for the stalemate, the most obvious is the lack of a means to effectively communicate between the different levels of command and between the front line and supporting artillery. Telephones were used but the wires were easily cut by artillery and often units were reduced to using runners, which caused a delay of hours if the message got through at all. Radios existed but they were too bulky for use in the trenches. Artillery was forced to fire by meticulously developed fire plans, but they did not have the flexibility to shift targets based on the tactical situation, this deficiency would not be remedied until after the war when radio was perfected.
Another shortcoming was the lack of battlefield mobility, even when a breach in the enemy line was opened the artillery that allowed the breach to be made was unable to support a continued advance because the ground was so broken it could not move up into new positions. Even cavalry had trouble advancing through the mire and devastation of no-man’s land. Until the advent of the tank in 1916, armies had to rely on infantry alone to sustain the advance, and this they could not do in the face of prepared defense as developed by the Germans.
Lastly, the Germans chose stalemate, the picked the line they would retreat to after the Battle of the Marne and they chose the most defensible ground in northern France to hold. They chose the defensive in order to free troops up for use in the east in hopes of knocking Russia out of the war. Between First Ypres in 1914 and the Ludendorff Offensives of 1918, the Germans only launched one major offensive in the west at Verdun, the rest of the time they were content to remain on the defensive. In consequence of this, the Germans developed one of the most elaborate defensive systems in history while they proceeded to try to knock Russia out of the war. They did not resume the offensive in the West until the spring of 1918 when an additional million troops were available due to the Russian collapse.
Stalemate in World War I was not inevitable, but it was probable given the mix of military systems available and the state of military doctrine at the outbreak of the war. The wonder is that so few generals understood the implications of the technology and tactics they would employ. There was ample forewarning in recent wars that pointed the way, most importantly the Boer War and Russo-Japanese wars.