Japan

Nanshin-ron vs Hokushin-ron
The national focus on northern vs southern expansion are unlocked via interservice rivalry decisions which in turn require Five Ministers Conference national focus. (Any single army priority decision can enable northern expansion on day end; similarly for navy priority and southern expansion.)

Tips

 * The large quantity of research bonuses from Japanese National Focus may prompt the age old bug of Hearts of Iron IV 'forgetting' existing research bonuses to trigger. When it triggers, some research bonuses will be forgotted upon reloading the game. There is no way to solve the issue on the side of the player. However, in order to mitigate the impact, the player can take advantage of the fact that research currently undertaken will not have an already applied bonus taken away. For example, a mitigating solution would be: a) to keep a sequence of old saves, not too spaced apart from one another; b) whenever research bonuses are found to be dropped, load an old save where the bonuses have not been dropped, change a research slot to research a tech to which the bonus in question applies and to which the player plans to apply in the future; c) save the game, and switch the research back to the player's original arrangement with no un-pausing necessary in between; d) do not over-write this save until the bonus is used; save to new files instead in future saves. Now the save will keep the bonus even upon game client reload. This solution retains the bonuses and the original plan of the player at the cost of losing some gameplay time.
 * Do not be tricked by the starting number of destroyers. Many of the existing destroyers do not have depth charges and thus cannot protect convoys from submarines. They are also short ranged.
 * Most of the IJN CL 1 (those with Long Range) have 2.00 Minelaying. Those CLs along with Wakatake and Mutsuki Class DDs have the range to reach far into the Pacific and reach UK through docking rights. They can used to mine English Channel for example well before conflict with Allies due to the likely early war with one or all of the Chinese warlords and ROC proper.
 * The potential capital ship armor bonuses from government and doctrines for Japan are as follows: Capital Ship Doctrine 6.5% + Decisive Battle 3% (locked in and cannot change) + Kure Naval Arsenal 5% + Nobutake Kondo 5% + Soemu Toyoda 7% (requires Battleship Focus)
 * Kure Naval Arsenal requires 2 additional dockyards in Chugoku. It is worth timing their construction to coincide with the completion of New Naval Estimates in order to maximize early game ship production.
 * Note: one IJN admiral does have the Ironside trait which gives a sizable 10% capital ship armor, though he is not the highest level among IJN admirals.
 * Beware that Battleship Focus requires Ultimate Battleship on the left most of National Focus tree. Similarly Carrier Focus requires Carrier Warfare Experiments.
 * Torpedo Cruisers from National Focus are cruiser hulls with higher strength and cost slightly more to construct than regular cruisers. A Torpedo Cruiser of a given tech level requires unlocking of the corresponding regular cruiser tech at the time of completing the Torpedo Cruisers National Focus.
 * As with vanilla HOI4, Marco Polo Bridge Incident modifier can be avoided by direct war justification. Beware, however, war justification may prompt AI UK to guarantee China.
 * BICE puts heavy emphasis on the difficulty with supplying troops far across oceans. With Japan, however, the modification has the side effect of drastically increasing the convoy requirement for internal trade to the point of pathology. Play need to choose between being conservative with invading minor nations or spending several years worth of naval production on convoy. (The latter is probably not enough. In any case, the player can of course modify SUPPLY_CONVOY_FACTOR for their own use. Mixing supply and internal trade is an artifact of vanilla HOI4.)