The 'First Battle of Gensoukyou' (Japanese: 幻想郷の最初の戦い; Hepburn: 'Gensōkyō no Saisho no Tatakai') was a major conflict between the humanoids and youkai races, taking place during the events of Touhou Reiiden: Highly Responsive to Prayers in 1686.
The youkai were being pests to the humanoids, and as a result, the Hakurei Shrine kannushi, Reimu's father, declared war against the race. However, during the battle, the Hakurei Shrine was destroyed by the youkai race's commanders, Konngara and Sariel. The kannushi was inside the shrine when the impact occurred, and when Reimu tried to get him and her brother to respond, they would not respond.
The war ended after both perpetrators in destroying the shrine were killed and having their souls sealed permanently.
Battle with Shingyoku
Reimu encountered a transgender youkai known as Shingyoku who was the gatekeeper to the Netherworld and Demonworld respectively.