They started spending that money after starting the war, so it's use is related to the war in question. Thus, when they will run out, whatever they were paying for (war related) will stop getting money.

It might not be a direct financing of the battlefield activities, but while the victory will be in the battlefield, the biggest chunk of the battle actually happens in preparation and logistics.

In other words, I'm hopeful that this will have a major impact on the invader's ability to cause harm.