I'll be the first to admit I'm not a financial guy, but how are those differences responsible for a rescued economy in the past vs. a sunk one presently? It seems to me an economic drain such as war, where products are destroyed without financial return is a net loss regardless of the factors you mention.
There are two economies at play in a war. The corporate economy which gets checks from Uncle pretty much daily to build and ship the implements of war and second, there is the Federal economy which writes those checks.
One reason the current war is not helping the economy is that [and I am VERY happy to type this] we are not in constant battle on many fronts. We have many small skirmishes but we do not have the kind of war that costs bucket loads of money on new stuff. The money we spend on IRAQ goes to troops, sustaining their equipment, maintaining Carrier fleets, Air Bases, Air time, etc.
In WW2 we would fly 1000 plane sorties where 150-200 planes would be shot down in one day. Those all had to be replaced. Factories were working three shifts to do that. We had major tank battles which destroyed thousands of tanks, artillery, and support vehicles, ALL which had to be replace ASAP. We had navy battles where carriers were sunk, planes shot down and entire infrastructures destroyed. That ALL had to be replaced. That provided jobs and money to workers and companies as they built and rebuilt planes, trains, tanks, ships, jeeps, carriers, everything to support 13 MILLION troops [we have 160,000 in Iraq].
That's the short answer