[cturner]

There's a lot of room for innovation here, and there hasn't been much precedent for this historically.

You could have constitutional measures controlling the way that wars were funded. A simple example: simple outlaw any kind of deficit spending by the national government. Another: a mechanism by which the release of funds could be tied up under legislation supervised by a third party such as the judiciary. [You could have a war chest, but encode it in a law such that the money could only be released if a senior court ruled that the release satisfied self-defense criteria.]

One of the reasons that the original United States constitution (Articles of Confederation) was replaced by the current one was related to military funding. In the old system, it was difficult to raise an army for shared purpose (e.g. fighting the revolutionary war) and there was potential for states to go to war with one another. The issues raised by that period will be familiar to anyone following European integration projects. Some hard-line libertarians in the US remember the Articles of Confederation fondly, because of the limits it imposed on government power.