In addition to the other variables that play into aggressiveness and/or duty performance while under fire, e.g. fire and maneuver on the enemy to destroy him in close combat, one needs to examine the conditions upon which soldiers enter and leave the combat theater.

Regardless of the overall national strategic goals, most soldiers will tell you that they want to, in order, 1. survive their time in combat and go home; 2. perform well in front of those they respect, mostly peers, but also superiors and, in the case of leaders, subordinates; and 3. win.

As you compare the conditions upon which a soldier can go home, disregarding medical or disciplinary reasons, look at WWII vs Vietnam vs GWOT. In WWII soldiers went home when the war was won, thus they had a personal stake in victory. In Vietnam, under an individual year long rotation system, and GWOT, under a unit based yearlong (in most cases) rotation system, the personal stake of the individual soldier is survival for a year. Leaders recognize this and, as anyone who has been deployed recently, try to combat both complacency and the reluctance to engage the enemy when terms of victory are ill-defined at best.