It's very simple, and is a further a check on the power of governement.
-Governement has no money, nor should it.
-If government wants a war, then it must gather volunteers to fight (over 1,000,000 volunteered after Pearl Harbor was reported to be attacked). Thus, wars supported by the public have no shortage of volunteers, but are generally confined to situations in which we are attacked, which was the position of the Framers of the US Constitution anyway.
-To fund troops, government should similarly only have use of funds voluntarily donated by its citizens. Afterall, if volunteers are willing to face death by way of military service (which is historically abundant for just wars), then they will offer money as well. Moreover, economic support is historically far greater, as it includes those who do not or cannot do the actual fighting, but who would like to contribute nonetheless.
-Historically, wars have been paid for by government bonds, but it should be noted that this is not the best way (imo). Bonds are coercion becasue future taxes are used to repay these debts. Thus, while far better than printing money out of thin air to monetize the debt or borrowing from a foreign source, I believe all wars should be financed by voluntary contirbutions, just as we donate to Ron Paul's campaign voluntarily. Afterall, governments do not go to war, people do. If there is no funding for such, then I guarantee the people do not want the war. If they do believe the war is just, there would be no shortage of donations of time, energy, labor and money.
Note: A war hawk (or even a good faith patriot) can argue this ideology is naive, but remember, it guarantees unjust, illegal, UnConstititional, liberty-depriving, weath-robbing wars will not happen. Further, by removing debt from the equation (since all funding is by donation, rather than debt), the ONLY historic beneficiary of wars (e.g., creditors) are no longer beneficiaries. Perhaps there would then be fewer wars as those accountable for the costs of any given war would be the only ones who could possibly benefit rather than a third party who has an incentive to encourage strife and misery everywhere for his own personal gain. In addition, if funding for a just war is too small (a fear that historically has no basis), then governement can always resort to issuing bonds, as has been done before, but by doing so, it must be acknowledged that this creates an economic incentive for someone else to encourage war (generally someone who bears no costs associated with the war).