OBL doesn't really believe he can -- or even care about -- defeat of the US. He cares more about having us withdraw from the world and stop meddling in corners that he considers his and his ilk's lands. The way he said he could see this happening is if we stumble like a drunken giant and break our own bones through acts founded in imperial hubris. He is hoping we do like the Soviets and overstretch and waste economic resources. Attacks that give momentum to such missteps are his goal and his idea of a "better" confrontation with the US since a direct eye-to-eye fight cannot be won. To this end, he knows small attacks by themselves won't mobilize the US to act like a drunken giant and hurt itself sufficiently enough that it feels better off withdrawing from exerting global influence in lands afar. [It takes great shocks -- perhaps even the product from a rapid succession of smaller attacks -- to get giants to jump. He knows that.]