The problem is you're putting the cart before the horse.
We no longer live in a Republic but a totalitarian dictatorship. the President wages war at the expense of you and I and this is because he hasn't bothered to get our consent before waging war.
In the law, the President cannot act at all unless your representatives declare a formal state of war.
The President is only allowed to move our forces when there is an impending invasion.
I would agree that this might be one of those times. However, one has to wonder why one would stage soldiers in Afghanistan leaving the nation wide open for an invasion.
On the matter of consent. First, the income tax takes all issue of monitary consent out of the picture. The income tax is in fact unconstitutional if indeed it really applies to all American citizens.
Second, it was specifically stated that one could refuse to serve. That's the law. If for instance, the war was to be waged on foreign soil and had no relationship to national defense, the potential soldiers could tell the feds to pound sand.
If the military were only funded for the two years that is demanded by the constitution, every incursion would require consent by those serving. And this is all designed with the fact in mind that all political and military power comes from the people.
I am sorry that this pains you so. Actually no I'm not because you really don't give a rats ass whether or not our military and foreign policy is either constitutional or libertarian in nature. You are seeking "libertarian" principles with which to justify your position. There aren't any.
I've lived under not one but two totalitarian dictatorships and can tell the difference between a dictatorship and a representative government any time. We have an elected government; nobody prevented the voters from voting in Harry Browne.
Here's the question for you. Imagine that the President not only gets a declaration of war from Congress, but also puts this "war on terrorism" to a plebiscite. I think you'd agree that the results of the prebiscite would be 90% pro-war and 10% anti-war. Now, these 10% do not consent to the war. Should we not go to war because of the 10%? How about the rights to self-defense of the 90% -- they choose to exercise them by empowering the government to fight a war? How do you propose the 10% withdraw their consent -- what in practicality they should, under natural law, do?