In 1914, artillery was sited in the front lines and firing over open sights, officers were riding horses and carrying swords, infantry were wearing red pants, the basic unit of maneuver was the company, and by 1915 everyone was mired in trench warfare.
In 1917, out of whole cloth, they had invented tanks, combined arms, the use of wireless communications, the use of indirect artillery fire (the British had experimented with it in South Africa to little effect), sound ranging and flash spotting for counter-battery fire, trench raids, armoured cars, the infantry squad, the light machine gun, the sub machine gun, the steel helmet, body armour, the trench mortar, the hand grenade, and tactics to use all that stuff.
All these things were developed over the course of about two years, which is a pretty short time when you think about it.
There was really nothing so revolutionary during the Second World War in terms of tactical innovation, not that comes to mind immediately. Tactics certainly evolved, but more gradually. Certainly the development of artillery usage in the Second World War was a major step forward, and amphibious tactics was also a major development - but I still don't think it matches the enormous learning curve they had in the First World War - from trench warfare to - dare I say it - "blitzkrieg" - in two years.
That was anything but flat.