But could not have been doing so during the war.
It sounds like a pretty simple principle: if a nation is suddenly producing lots of guns and bullets instead of peacetime industrial goods, and has a finite number of industrial workers (women aren't superhuman; you can't train up millions of women to do a new job instantly, just as you can't do so for men), there will, inevitably, be a decrease in the amount of peacetime industrial goods it can produce. This provides an opening for foreign countries to supply it with those goods.
War hurts the nations that partake in it, wasting money and human lives on the task of killing people, and helps the nations that stay out of it and stay undamaged. I wasn't aware that this was controversial.
Perhaps I'm just being ignorant.
No not ignorant...
Its clear that Germanys output of "consumer goods" will drop during the war - but this does not mean that a foreign supplier is immediate compensating the reduction of supply.
On average the lack of supply will be upset partially by a reduction in demand - during "hard times" people tend to spend less - things that will be replaced during peactime will be used for additional time during war.
Household income is probably also reduced on average, so less money is there to buy things, imported goods will be more expensive too (especially if the wartime nation suffers from inflation + transport cost)
the warring nation will also import more likely more war material than consumer goods. - It will face reduced exports too. maybe - redirecting production of those exports towards goods needed domestically (including war material)
In sum the foreig nation will probably be able to increase experts to the warring nation, but not fully compensating the production drop. It might also replace the warring nations experts to third countries (Eg. if Germany sold tea kettles to spain during peacetime, it will likely not do so in war as the tea kettle production is reduced to levels to supply the domestic market. So Britain will be free to take over the tea kettle market in Spain, but not in Germany.
After war Germany might have troube to again export to spain (in the same volume like pre war), but some Spaniards might favor Krupp Tea kettles to brit teakettles after all
War hurts nations - definitely, but it also does it not only at the supply side - you have to consider demand.
Sure Britain will be better off, but probably not on a 1-1 base (750.000 - less dead does not mean 750.000 more employed) also less dead mean more domestic consumers = not all additional product is being available for export...