A corollary of the measure/counter-measure dynamic is the indirect approach. If your opponent is trying to outwit you, avoid doing what's expected. Rather than blasting through the Maginot Line, the Wehrmacht defied expectations by invading France through the Ardennes Forest. Instead of fighting a delaying action against the much larger Union Army, just landed eastern Viriginia during the 1862 Peninsula campaign, Robert E. Lee went on the attack. (McClellan's amphibious invasion was itself a bit of a surprise--particularly given the numbing predictability of previous land-based Union assaults in northern Virginia.) In Operation DESERT STORM, coalition commanders did a double-feint: they encouraged the Iraqis to believe that the Americans would do an end-run amphibious invasion, and then took a direct path through the desert instead.
Basil Liddell-Hart, in his book Strategy, is the most famous advocate for this notion (and he gave it the name the indirect approach). Luttwak discusses it at some length, too, as a way of showing how important it is to keep the enemy's state of mind and likely actions in mind when crafting a strategy. To go back to the example of a family trip to the beach, if you think someone is actively trying to stop you from getting there, take a roundabout route through the back roads--not the well-travelled, expected route. Luttwak uses the phrase "the principle of efficiency" here to highlight the occasional ironies of military strategy: while the direct route may be the most efficient, it may not be the most effective.
The concept of the indirect approach isn't hard to understand. It's a lot trickier to pull off. Strategic deception, which lies at the core of the indirect approach, isn't easy. It took a massive deception campaign, including mock inflatable tanks, to convince the German high command that the Allies were planning to invade at Calais, not Normandy. One well-placed spy, and the ruse would have exploded like one of those tank-shaped balloons.
There was nothing wrong in principle with a hurry-up invasion of Iraq. Many observers were worried that there weren't enough troops and equipment in theater yet to win a quick victory over the Iraqis. As in 1991, the most worried analysts proved to be wrong. The Iraqis, too, seem caught not totally prepared for the US invasion. Despite the months-long beat of wardrums, it was hard to believe that the United States and Great Britain would invade as quickly as they did.
The problem, of course, wasn't invading faster than expected, with fewer troops than expected. The real mistake was occupying with fewer troops than needed. The dynamics of the indirect approach work best when your opponent is a single leader, or a small group of commanders, who stand to win or lose a conventional war by which troops they position where, with what orders. At some point, the nature of the conflict changed, from conventional warfare to counterinsurgency. On that day, the number of troops did matter in a new, critical way. In the day-to-day operations against urban guerrillas--often which boils down to simple police work--the number of people who can patrol, interview, monitor, ambush, and do the other tactical jobs of counterinsurgency matters a great deal.