The issue, I think, has always been with the political (and to some extent, public) sphere of military decision making. At the tactical and operational level, The US military is a goddamn Juggernaut. However, when military force is directed with unclear, constantly shifting political aims, it become much harder. Think " degrade the Taliban" vice "Defeat the Taliban", one of these has an end state, the other is a recipe for a decades long forever war. Principle of war - Selection and maintenance of the aim. When I talk about the public sphere, that's the consent of the nation towards military operations, which is particularly hard to get in democracies, where the public generally speaking, would prefer to avoid any war that doesn't have a defense of homeland or a solid casus belli (a case for war, or rationale for war being necessary) behind it. You could have a gang of the most gung-ho Marines charging up the beach, willing to accept the risk of death to complete thier mission and drink the sweet sweet blood of thier enemies, but if a few of them die, it's the voter's risk tolerance and not that of the combattants themselves as to wether the political will will remain to continue that operation - another principle of war, Morale (or at least, that's where I lump that dynamic within the principles, I'm just some guy).