To quote Julian Assange, "The goal is to use Afghanistan to wash money out of the tax bases of the US and Europe through Afghanistan and back into the hands of a transnational security elite. The goal is an endless war, not a successful war".
But I would point out to you that the military's budget is $700B regardless of whether the US is engaged in foreign armed conflict or not, and half of that is personnel costs like salary and housing allowance. Only about $300B goes to procurement of all types (weapons, clothing, food, cleaning supplies, fuel, paint, etc...)
On the other hand in a situation where warfare is considered with less weight and investment feels like it’ll embolden states to return to a state of frequent warfare.