Earlier quoted context omitted.
I don’t think they have the economic resources to sustain a prolonged engagement or occupation.
Isn't their capacity to maintain an ongoing assault about available meat sacks, ammunition, food and oil - none of which they lack - rather than money?
There have been mentions that they're already short on food for their soldiers. Wonder if that's an actual thing, and if so, then how badly short they are?
Especially with that old phrase "an army marches on it's stomach" coming to mind. :)