At the beginning of WW1, modern armies lacked any effective mobilization tactics. For millennia, the tactics of war were centered around outflanking your opponent, forcing him to retreat or be destroyed. That was mostly accomplished by cavalry, who could exploit openings in enemy lines and quickly move where they were needed.

In WW1, men had machine guns. Good ones, cheaply made. Lots of them. It's easy to hit something as big as a horse with a machine gun, and very hard to protect the horse against it. Suddenly, cavalry were of very limited utility in war. Artillery bombardments created cratered moonscapes of forests, further lowering movement speed for both sides. An inability to advance created a need for entrenchments, to allow soldiers to get some form of rest and protection while they slept if nothing else. These defenses grew bigger, and bigger, and more complex, until they were essentially impossible to breach. Indeed, Germany lost because they ran out of skilled soldiers to man the defenses, not because the defenses themselves could be breached if held properly. The world's first tanks only rolled onto the scene in the final year of the war, once Germany had essentially lost already, and airplanes were good scouts but not powerful or numerous enough to make a serious impact.

By the time WW2 came around, tank and airplane technology had advanced immensely. Tanks are faster than horses, heavily armored, and pack a huge punch, but aren't very good at holding territory. Airplanes are even faster and provide amazing reconnaissance information, allowing generals to quickly and accurately sketch out sweeping plans and carry them out.