Major war time supplys?

Does anyone have any idea of what Goverments would do in a major war that had say been dragging on for a year or more,if all this goverments airforce were to have been destroyed do theygo without them or do modern goverments have contingency plans say for example to start the production lines up again which would in any case take a long while or do they buy from other countries?I know it sounds like a stupid question in a way but i often wonder if the whole of the world was at war like in WW2 what would their plans be.I wonder if they’d go back to producing very simple aircraft and simply mass produce them.anyone got any thoughts on this subject?