Farming in remote areas. Cities are death traps when a full scale war happens and they lose supplies, it's the same story every time. A typical picture is of some city dweller trading their valuable jewelry for some potatoes. Most recently it happened between Crimea and Rostov-on-Don, for example Mariupol.

Also every time there's a major war between roughly equal forces people rediscover all kinds of things: there aren't nearly enough munitions, there aren't enough bunkers, trenches are overpowered defensive structures, we need more water and food and fuel, air dominance or at least air denial is critical, tanks and mechanized/armour units are actually very good, etc. etc.