@ Laundrysauce Based on WW2 statistics (which i know is greatly outdated), Japan would last roughly 2 years, half that in wartime. If you factor food, roughly 3-5 months.
They produce their own food and other necessities however it's greatly lacking consumption. Japan pretty much imports EVERYTHING.
Critical resources they import are scrap metal, rare earth, oil/energy, and food.
Even the US would only last 11 years tops (energy, based on oil 2017 reserves (not including the hidden stockpiles every country has)) if they lost all those imports, and that assumes they wont lose key infrastructure after a year or two. Can be damn sure that multiple aspiring leaders(dictators) will branch off and create their own military factions too if given the opportunity once they are transported away from so many obstacles. You get real close to your own branch and being ordered to give your now non-replenishable supplies in an order you don't agree with can cause massive friction.