AFAIK, this is how it goes:
1) Duchy of Crest, led by Eskatia, launched an invasion against Solar Kingdom, led by Verlaat. This was done after much persuasion from Annarose.
2) Verlaat asked assistance from an emerging mercenary nation, Heierrat, led by Yuni.
3) With Heierrat's help, SK managed to easily repel the invasion.
4) The failure in the invasion proved to be fatal for DoC because they have used too much resources for their invasion. Knowing this, Verlaat used the opportunity to strike back.
5) Again, Verlaat asked the assistance from Heierrat. But apparently, Yuni made the policy to only help a defending nation. She declined.
6) Still, due to the superiority of the force of SK, DoC fought a losing war. Eskatia, concerned for her people, and Verlaat, not wanting to burn more resources than necessary, decided to negotiate for peace. This ended with their marriage. The marriage also marked the end of the War. This also means, in a sense, DoC became the subsidiary of SK.
7) Because of the recent success and TGC's influence, Verlaat envisioned an empire under his rule in the realm of Lavato. This is, in a sense, his method of achieving peace. This ambition was also reasonable because his kingdom by this time was supported by Duchy of Crest, True God Church, and his indebted ally, Beltemat Kingdom (not Beltorat). Power makes ambition.
8) Before he started his campaign, Verlaat decided to invade Heierrat. Because of Yuni's policy, Heierrat could and most likely would hinder his conquest. This should benefit him in the long run.
After that, I still don't know the outcome of Verlaat's invasion against Heierrat, nor do I remember what happened with Alphonce... I predicted this is what happened. There is no source for point 9 and point 10 whatsoever because I don't know Japanese
9) While Verlaat concentrated on Heierrat, Regus Kingdom and a young kingdom, Beltorat, allied themselves and launched a joint attack against SK. This was done in the name of avenging Heierrat. Remember that Yuni made a peace treaty with all kingdoms and promised an assistance if any of them defended against an invasion. This treaty was used as a reason for Beltorat and Regus to invade SK.
10) While SK fought a more-or-less stalemate battle with the three allying kingdoms, the envoy of chaos started to reveal their true selves and their ultimate plan of inviting Elgandi to Lavato.