Got into the new EU4 expansion Mandate of Heaven. The game in Japan starts during the Onin War so the whole nation is basically a battle royale between 20+ different daimyos. This really showcases the best feature of the game and why I've sunk hundreds of hours into it - parsing the shifting alliances between other states to avoid conflicts with strong nations and isolate and attack weaker ones.
Starting off as the Shimazu on the South island Kyushu, first target is my neighbour Ito, but they are allied to the Hosokawans, a strong faction based in Osaka, so I can't declare war on them or Hosokawa will enter the war on the side of the Ito and destroy me. But, because Kyushu is connected to Honshu via a straight, I can put a fleet of ships between the islands which would block Hosokawa from crossing their army while I take out the Ito. I take out some loans to build up my fleet to be approximately the same size as the Hosokawans, put an admiral in charge and hope that when I declare war on Ito, my fleet wins and can continue to block the straights. I declare war on Ito and - Hosokawa dishonour the call to arms by their ally! In a straight up 1 v 1 fight I capture Ito and double the size of my lands to 2 provinces.
Next up is my neighbour the Kikuchi. They are allied to the Yamana, another big daimyo in the centre of Japan, and the Ouchi, a small daimyo next to me. I don't want to declare war on the Kikuchi and have the Yamana dragged in. I also don't think I can replicate the fleet trick with Hosokawa because there is no way I can defeat the combined fleets of the Kikuchi, Yamana and the Ouchi. But, if I go and look at the diplomatic relationships of the Ouchi, I see they are allied to the Kikuchi and the Hateyama - a small clan in the north of Japan and no threat at all. So to take Kikuchi, I actually declare war on the Ouchi, The Kikuchi enter the war as the ally of the Ouchi, which is my goal, but the Yamana are not called in. I can easily take the two small armies of the Kikuchi and the Ouchi and my small daimyo grows again.
The last roadblock to taking the island of Kyushu are the Shoni. I allied with them early on when I was only a one province minor, in case I needed help in a defensive war. I've since dissolved that alliance and can declare war on them. Again, the problem is they are also allied to the Yamana who I don't want to go to war against. The Shoni have no other allies I can enter a proxy war against either. Seems like a stalemate. But then I notice the armies of the Yamana are on the march - I check their diplomacy tab and yep, they are at war with the Hosokawa. Even better, the Shoni have entered that war as well and march their troops off to the main Honshu island. Now it's a simple matter that I can declare war on the Shoni, and the Yamana will not heed the call to arms as they are already engaged in another war which lowers their willingness to honour alliances. With the Shoni forces already decamped, I occupy and annex them with little trouble. Kyushu is mine.