Best Military Concept?

I've been watching this "Military Top Ten" documentaries. They're about the top ten tanks, top ten fighter-planes, top ten warships, etc. What if we don't limit ourselves to weapons of destruction? Top ten weapons or strategies or training schools or concepts of any kind.

Give a countdown of 10, or just your top #1, or anything in between. As a former Sergeant of Marines, here's my top 3:

3: Artillery. It's not called the "King of Battle" for nothing. Artillery is responsible for about 60% of all casualties in all wars since its inception. Air power, battleships, infantry, tanks, and everything else other than artillery accounts for the other 40%

2: Drones. Never have so few risked so little to kill so many.

1: The Camel Back. 100 oz of hydration easily carried on your back (even with a full pack and/or body armor) available via a mouth-activated nozzle that hangs over your shoulder. The second best thing about water: add sugar for extra calories when fire watch stole all your rations on the first night of "The Crucible".