The US military is NOT a paper tiger. It is not the fault of our people in the service that the leadership in this country received it's collective military knowledge from old John Wayne movies. These troops (for the most part) are intelligent, physically capable, and extremely courageous...but they not superhuman enough to conquer a foreign civil war with less than 200,000 American forces present. To really handle something like this would require an increase to 250-500,000 troops, which in turn would multiply US casualties 10-20x. That is the number-one reason why this Administration can not admit to a civil war there...to do so would require the above, and would be even bigger trouble for our leaders and their toadies.
Just like in Vietnam, the opposition is allowed to control the action. Our American forces are neutered and forced to make due with coninuously failing policies while the civil war is able to fester & rage and Iranians filter into the country just as the North Vietnamese did.
If the day does come when we are involved in something of a World War Three, we should have no fear in the abilities of our soldiers, only fear that such an event may occur under leadership as inept and cowardly as that which we currently have.