The mess of WWI came about in part because of old military tactics being used but with new weapons that were much more deadly. What do you think was the most significant new weapon in WWI and how did it contribute to the length and overall destructiveness of the war?
I think that the tank was the most significant weapon of World War I. The tank had the ability to end the stalemate of trench warfare, because it could cross the trenches, and survive machine gun fire. If tanks had been brought into combat earlier in World War I, they could have prevented many deaths, and shortened the war.
Overall, I'd say the water-cooled, belt-fed, machine-gun was the biggest game changer in the Great War. That followed closely by artillery. Its true at the end the tank, the aircraft, and even lighter automatic weapons begin to play a significant roll, but "the devils paintbrush" (the MG) was was really made the war what it was.