War being expensive is especially ruinous with modern (ie anytime past the 18th century AD) countries being deeply interconnected through market systems. All that money just pissed away trying to grab someone else’s land? Every other country in the world was investing that money in material or human capital incentives. When peacetime returns, unless you have a massive waiting market for your war economy production (like the US after WW1 and WW2), you will be at a massive disadvantage in the world market - causing a large number of domestic industries to suffer as they’re undercut by products of either superior pricing/availability, quality, or both.
At this point, you can swing towards protectionism - but that will reduce the standard of living and efficiency of domestic industry as well, and potentially lead to backlash from other countries imposing protectionist policies in kind, damaging your exports and leaving your nation even poorer. So at the end of the day, no matter what you do, you’ve fucked over your own industries and budget for years at minimum, requiring significant political capital and competence in government just to make up the difference and catch up to the rest of the world - and that’s not even getting into damage done to diplomatic reputations, actual loss of materiel, and the sheer human cost of throwing people into the meat grinder.
War requiring a war economy is a no-win scenario in the vast majority of cases.