The U.S. Department of Defense is changing the way it plans for future wars. For over half a century, war planning was done with a lot of knowns. During the Cold War, and even before and during the two World Wars, a lot was known about where the enemy was and what they had. But now things are different. Terrorists are a bundle of unknowns, as are many potential future wars, or peacekeeping operations. So war planning has been changed to regularly deal with a lot of surprises. Well, the purpose of war planning is to reduce the degree of surprise. So the 21st century war planning will involve playing, literally, a lot of what if? games and exercises. Getting used to anything goes is not easy for a military institution that has spent a century planning in a less chaotic environment. Military planners expect the next few years to be rocky, as old methods and traditions bump into the unexpected.