Basically it happened between RC1502 to RC1509.

Its prequel was the increasing shift into capitalist economies, abhuman rioting against their abuse and other events which meant many nations had to re-evaulate how to treat their abhuman minorities (ofc it was no problem for weird countries with abhuman majorities).

The Novogrod Empire which ruled most of Russia adopted new regulations which put abhumans and especially beastmen into borderline slavery. A massive wave of beastman refugees fled to the West. This included East European nations (which back then were not part of the League) and especially the Varangian Federation (basically an united states of Scandinavia). The Novogrod did not like this at all and demanded these nations to refuse taking in the escapees. Meanwhile the flood of millions of refugees caused a rather stressful atmosphere within Varangia, enough for certain extremist factions to gain power and take much of the Federation. In spite of that there was also a similarly strong opposition and things soon turned into a civil war.

In the following months the international atmosphere grew polarized, most powers taking sides at supporting either party of the two. Meanwhile the Novogrod Empire declared war on the small nation of Rutene. They happened to be allied with the Hungarian League thus the nation suddenly found itself facing off against the giant Novogrod Empire. At the same time the Novogrod held secret talks with Jodesian leadership and formed a military pact. Having its hands full with the Novogrod the Hungarians were unable to aid their ally Franco-Iberia when Jodesia launched a surprise attack on them and the Southen African Coalition.

And well, this pretty much started a chain reactions with practically every major nation choosing a side in this conflict. Feeling the push of the then rich and highly developed Jodesia the Franco-Iberians requested Matagasy help. At around the same time the Han Dynasty joined the Novogrod side thus the two empires proceeded to clobber each other as always. Casablia may also support the Han Dynasty due to sharing the same anti-abhuman beliefs but it's up to the player. NAU would support both Jodesia and Franco-Iberia and make a good dollar out of it. Given the fact the main point of contention is the treatment of abhumans, Oceanos with their almost pure beastman population would support the "Allies" against the "Novogrod Confederation". Things get messy but with more and more nations supporting the Allies the Novogrod is gradually weakening and eventually collapses by late 1508. It splits up to numerous small nations, some of them managed entirely by abhumans. Jodesia keeps fighting on and so does the Matagasy and Han, the latter being pretty much consumed in their own private war. Eventually Jodesia capitulates and loses a sizable portion of its territories to Franco-Iberia. Meanwhile the Hungarian League takes the remaining East European nations and the Finnish portion of Varangia under its wings as their "protectorates" (becoming members in 3-5 years). The war between Han and Matagasy went on but without a satisfying conclusion to either side. The enmities never stopped since then.