Prior to the Hundred Year War, the city was an important center of commerce. However, it was destroyed by the Fire Nation during the first wave of attacks due to its strategic location and importance to the Earth Kingdom.
Failed invasion of the Fire Nation capitol by combined Water Tribe forces and allies of Avatar Aang
A small team of allies of Avatar Aang (Earthbending Master Toph, Ambassador Sokka, and Head of the Kyoshi Warriors Sukki) infiltrate and destroy the Fire Nations Air Navy.
Then Firelord Azula defeated by Prince Zuko and his second, Master Katara, in Agni Kai. Firelord Azula broke the Agni Kai by attempting to attack a non-participant and was subsequently defeated, forfeiting her right to the dragon throne.
Whaletail island was chosen as both a symbolic location near the Southern Air temple, where Avatar Aang is from and where the war began, and as a neutral, uninhabited and unclaimed island, for the peace accords following the Hundred Year War. Fireord Zuko of the Fire Nation, Avatar Aang of the Southern Air temple, Chief Hakoda representing the Southern tribes and Chief Arnook of the North, as well as Earth King Kuei and the council of Generals met here to finalize the ending of the war, and to move forward with reparations.
The Harmony Restoration Movement was a campaign to remove the Fire Nation presence in the Earth Kingdom by transferring the Fire Nation colonials back to the Fire Nation. It was proposed by Avatar Aang at Earth King Kuei's re-coronation ceremony and was greeted with cheers from the Earth Kingdom citizenry. However, the movement eventually led to a conflict between Earth King Kuei and Fire Lord Zuko, mediated by Avatar Aang. The Harmony Restoration Movement ended with the creation of a new joint government in the former Fire Nation colonies which eventually led to the creation of the United Republic of Nations.
More of a standoff than a proper battle, Yu Dao was the center of conflict regarding the fate of the Fire Nation colonies post-Hundred Year War. The Battle for yu Dao was a standoff between Fire Nation and Earth Kingdom forces over the fate of the colonies, now the home of a combination of Fire Nation and Earth Kingdom citizens of mixed heritage who did not want to be forced from their homes.