There once was a temple that would transfer you to the gates of any village you wished. Even a mortal could use the temple, but only if that mortal had the keys to the gates he wished to travel to. This was the Temple of Gates created by the three goddesses to save them long journeys from the heavens. It is unknown how it got from the sacred realm to Hyrule Field.
The king of Hyrule once sent his knights into Hyrule field to search for the fabled temple. Many of which never came back from the infinite field. It was the noble Tingle who first found the temple and used his keys to return. The king developed maniacal thoughts about what he could do. He had an army, the keys to the village of Humans, and the village of Zora's, and he had a detailed map that showed him how to get to the Temple of Gates. Tingle, begging for a reward, was thrown out of Hyrule and the king sent his men marching South into the field.
The king enslaved the Humans. Survivors spread news of the kings actions and alerted the Zoras. Most Zoras left the village and hid in the Water Temple underneath Lake Hylia. Some skeptics stayed in the village and were quickly enslaved. A great war was about to begin.