Unlike any other nation part of the Concilium Mundi, Nihhon is entirely exempt from obligatory contributions to CM military operations or agencies. Independent arms research by Nihonia is permitted and is ongoing, but all research must be fully submitted to the CM's databases.
Nihhon's exemption is validated through the original peace treaty and terms of formal surrender (and revisions) agreed upon on December 13th 2067(1314) in Cynoto, the nation's former capital; a contract declaring Nihhon's perpetual neutrality. Due to this neutrality, embargoes on 'enemy' nations are side stepped to a degree by dealing through Nihhon markets for goods from other nations now denying sale. This unfortunately allows the formation of a sizable black market. Large 'independent' international CM investigator firms are a focus of the CM in Nihhon to both retain Nihhon's neutrality and prevent it from becoming a terribly effective safe haven/dealing centre for kaslati (terrorists) and the like.
The Jidai Calendar Era was set up during the first negotiations of surrender, however by the mid22nd century the AUC system had become more popular.
Some in Nihonia (especially those that had little part in the war) felt that Rome's invasion plan was sneaky and underhanded, but the legions and auxilia had shown enough courage and honour that they begrudgingly respected Rome's victory.
To try to support Nihhon's perpetual neutrality, part of Rome's investment and infrastructural aid contributed to Taoist daoguans and monasteries. Taoism was considered by the military occupation governor to be less confrontational than the previous state religion of Shinto-Buddhism. Shinto-Buddhism was not outlawed, but Taoism experienced a lot of encouragement.
Although Nihhon was barred from engaging in any military conflict, a self defence force was planned early on as Rome organised its formal withdrawal.
Nihhon was directly occupied by the Roman military until 2070(1317), at which time a provisional republican government with continuing Roman leadership was established. Nihhon's flag was changed from the Sanshu no Jingi based design to the chrysanthemum and rising sun at this time to accompany the new provisional government. It was a more peaceful but equally historic symbology. Nihhon became fully independent in 2074(1321).
Post-war Economic Recovery
Nihhon's recovery after WWI was fast, but not as quick as the OTL Economic Miracle. This was due to the fact that Rome's interests and investments centered only on preventing another militaristic regime from emerging while at the same time developing far-eastern allies. Rome saw that promoting economic development in Nihhon would help republicanism to spread, but without an ideological cold war, its priority was understated despite being addressed earlier and ahead of political reform (in comparison to OTL). Peace in the region aided balanced development, but had an equivalent Korean War occurred, Roman military investment would have helped recovery further. At least Nihonia was spared a recession from that non-existent war's end.
Nihhon's own governmental intervention played a vital role in the country's recovery, helping the continued modernisation of the nation.
In response to the growing black market and foreign businessmen, there was a growing native trend towards isolationism both culturally and economically. However, Nihhon's politicians were persistent in policy to keep Nihonian markets open and expand trade if not cultural openness.
The Minkokuhei became Nihhon's official currency in 2073(1320), included with amendments to the republic's initial constitution. Both the PAB's Yahuo and Rome's Carici are still widely accepted currencies, equivalent pricing is labelled on most products.
So, whereas Japan experienced an explosion of development from 1949, fully recovering by the 60's (~11 yrs), Nihhon's equivalent economic resurgence was from 2069-2089 (20 yrs), just in time for the Sinican Revolution.
Culture and Impact
The occupying garrison rotations introduced Nihonian culture to the rest of the Empire even while Nihhon society itself became isolationist to re-establish their identity and economy. The impact on Roman society was almost as great as the conquest of India (1734-1770's). One specific example was the word Osoji (Romans spelled it 'Osogi') which could be used to mean any sort of house cleaning but was normally used synonymous with the old Parthian Nowruz and Hebrew Pesach spring cleaning traditions.
Sasheipen, pulp graphic novels and serials also developed into their modern popular form during this time.
Nihhon's self imposed seclusion from global cultures since the occupation faded rather slowly over the next century.
The game of beidaitana (bedamitana, badminton) became extremely popular in Nihhon after the banning of sword practice. Two players (sometimes as many as six, in two teams) would be positioned in individual small conical, circular, or hourglass outlines a distance from each other. With tightly strung rackets and feathered cork or tero birdies, the opponents try to hit the opponent's court. There were penalties for leaving one's area, and being hit by a bird makes for a reset of serve. Games often moved very fast.