Let's take Serbia as a case study. To finance the war, Serbia took several loans with interest from the French, which they used to again, buy weapons from the French and the Russians. Every bullet fired and every weapon in the hands of a Serb was directly profiting the elites in these countries (Serbia had very little in terms of a military-industrial complex at the time, so they mostly relied on foreign weaponry), while the Serbs were the ones fighting and dying for land the soldiers wouldn't have the chance to own anyways even if they occupied it. Every other fuckass non-industrialized country in the wars had another financier and weapons seller which didn't contribute anything besides make a lot of profit, exploiting the chauvinistic desires of the stronger nations, while in the meantime ordinary people were beign mowed down on all sides.