“Opening up positions” to those that would not otherwise be hired [because assumedly they are less able substitutes] is a negative for the economy. If the army was composed entirely of idiots and invalids that were otherwise near unemployable, that would be ok. But that isn’t the case..until they come home damaged by the war, which is the other side of the matter. Now you have taken someone that would have been able bodied wealth creator and hurt them so that they are far less able.
The historical data and shown this over and over, being in a war screws up your economy. If you win relatively cleanly there is a bounce back effect, especially if it gained some economic advantage (via loot, political influence, market access etc.). But during the war and in period immediately following it the economy suffers. Swords or plowshares!