What caused the stalemate on the western front in WWI?
The stalemate along the Western Front during World War I, known previously (i.e., before anyone appreciated that there would be a second world war a couple of decades down the road) as the Great War, was primarily a product of neither side's ability to advance against the other side's intricate system of trenches that, combined with the deadly new weaponry being employed (such as automatic and chemical weapons), caused the fighting to stabilize into a series of charges and counter-charges of infantry into the teeth of the enemy's defenses. As the war dragged on, all participants, save the United States, a relative late-comer to the fighting, confronted growing shortages of soldiers, and each participant's national treasury was similarly depleted by the enormous financial costs associated with the protracted conflict.
The Great War was, prior to the outbreak of World War II, commonly referred to as "the war to end all wars" because of its scale and the level of carnage involved. The trench warfare that most characterized the fighting lent itself to the kind of stalemate that did occur. The introduction of chemical weapons, especially mustard, chlorine and phosgene gases, was a direct result of the war's participants' desire to break out of the stalemate. The cause of that stalemate, though, was the employment of the systems of trenches that, by their nature, lent themselves to the establishment of fixed positions. One result of this stalemate would be the development, primarily by German and Russian military officers, of what would be known as "manuever warfare," the use of tanks, armored personnel carriers, and trucks to navigate around the enemy's fixed positions (such as the infamous French "Maginot Line").