Complex sentence: Upon Nobunaga's death, Oda clan's grasp over former Takeda territories weakened, and neighboring Tokugawa, Hōjō and Uesugi clans all started to contest this vacuum of power in Shinano, Kōzuke and Kai provinces.
Split into simpler sentences: Upon Nobunaga's death, Oda clan's grasp over former Takeda territories weakened. Seeing this, neighboring Tokugawa, Hōjō and Uesugi clans all started to contest this vacuum of power in Shinano, Kōzuke and Kai provinces.