dont be friend with neighbour countries every country need a way to grow on map invade capital of mukden dont trust to player who control to japan ıf japan wants to win he must to capture korea firstly research ınterceptor and submarine put a few submarine around of your country research tactical bomber build air bases 2 to every province which ones has factories build fortress against manchukuo and start attack to japan with tactical bombers invade all ısland
after this especially keep to produce light tanks tactical bombers and secondly artilleries ınterceptors ıf u dont have enough resource to produce land units keep produce submarine and put them every point on sea
after japan u can start to fight with manchukuo and communist china send light tanks near of city ıf u see units stop to tanks and destroy them with tactical bombers and continue to march build air bases in captured non core provinces and continue to march

before your attack to japan you should protect your south coast from japan when attack time came and u have tactical bombers and submarines ıt depends to situation ıf japan has naval bases first u should control naval bases for enemy submarines ıf japan hasnt any naval base so ıt means japan hasnt any submarine u dont need to blockade his ports u can send your submarines to west and east side to cover from others while you are attacking to japan bomb with tactical bombers and send ınfantry hold a point in japan and build air base and fortress in there bring your tactical bombers to japan and bomb everywhere