The engagement, however, won’t in all respects follow the plan. Not only will its outcome depend on what the other side does—the “known unknowns,” of which former secretary of defense Donald Rumsfeld famously spoke41—but it will also reflect “unknown unknowns,” which are all the things that can go wrong before you’ve even encountered an adversary. Together, these constitute what Clausewitz called “friction,” the collision of theory with reality about which Artabanus tried to warn Xerxes, many centuries earlier at the Hellespont.
