I've posted it elsewhere, but Wano Kuni is meant as a reference to Tokugawa Japan (1603 - 1868). A closed country, led by an official called the Shogun, that has successfully repulsed incursion by the outside world (World Government in One Piece, colonizing European powers in real life). Wa is an ancient name for Japan, Chinese in origin. Wano Kuni means Country of Harmony (or peace), an old name for Japan. Japan's isolation ended in 1853 when a hostile, outside power forcibly opened Japan, imposing humiliating "unequal treaties" on the Shogun. A civil war followed, between supporters of the Shogun and Emperor. The War ended in victory for the nobles loyal to the Emperor, resulting in the Meiji Restoration. Japan quickly modernized, forcing the suspension of the unequal treaties, and emergence of Japan as a major power.
My hypothesis is that Oda will follow the real history. After the War, Wano Kuni was opened by force, under humiliating terms. Based on
Momo and Kin's reaction, there seems to be a connection between Kaido and Wa. What if Kaido's Smile Army was the equivalent of Commodore Perry's Black Ships? Kaido forcibly opened Wa to the outside world, imposed a treaty, claiming Wa as his territory? Kin, and his friends, are part of the resistance, loyal to the Shogun, but opposed to the foreigners. In real life that role would have been filled by the Shinsengumi.