Notice: This page is a work in progress. The Mushroom War was a rather short war that only lasted for a little more than a year and a crap ton of countries got involved for no reason. The war was mainly between Hammer Island and the Mushroom Kingdom.
Tensions between the Mushroom Kingdom & West Hyrule and the rest of Hyrule were always fierce to to past relations and the fact that Bowser owned North Hyrule. Hammer Island and the Non-West Hyrule nations had always been friendly with each other, but nothing was ever done with their alliance until February 1815. During this point, Ceiling Cat, the president of Hammer Island at the time, decided to invest in Lamp Oil so there could be more "clean energy" (even though Lamp Oil wasn't much better). He also invested in Rope and Bombs because f*ck it. Naturally, who else to go to other than Morshu? So, Hammer Island and East Hyrule started to strengthen their trade relations and became great allies, with Our Country trading their mass amounts of fruits and vegetables, especially PikPik Carrots, with East Hyrule in exchange for Morshu's goods. Up until this point, the Mushroom Kingdom and Hammer Island had been neutral towards each other, but the Mushroom Kingdom began to grow weary, as Hammer Island was now great allies with East Hyrule who was great allies with North Hyrule, the great empire owned by none other than Bowser. As they had feared, Hammer Island soon also became great allies with North Hyrule, as well as Southeast Hyrule and even South Hyrule, which at this point only existed in the future, as North Hyrule had given Hammer Island a time machine. This huge new alliance posed a big threat to both the Mushroom Kingdom and West Hyrule, as they were obviously enemies with the Non-West Hyrulian Countries. Finally, the Mushroom Kingdom decided to cut off all Mushroom trade with Hammer Island as a way to tell them to maybe chill. This pissed Hammer Island off, since the Mushroom Kingdom was the only place they could efficiently get mushrooms, which were considered legal drugs, so feeling confident, they declared war. North, East, South, and Southeast Hyrule declared war too, because they also benefited from Hammer Island and the Mushroom Kingdom's Mushroom trade. The Hyrulian Civil War was technically still going on, but only because Fake Robotnik hadn't finished signing the Hyrulian Treaty (and wouldn't finish until 2166), so it was more or less over and wouldn't interfere with this war. UnAmerica decided to help Hammer Island and Non-West Hyrule because they liked Capitalism, and South Korea also assisted because it was owned by Morshu. North Korea decided to help the Mushroom Kingdom and Hyrule's side for the sole sake of being relevant.