The Earth Sphere Unified Nation, or ESUN, was created in AC 195 out of a plan by the Romefeller Foundation to unite the world against the Colonies. The former leader of the Sanc Kingdom, Relena Peacecraft, was made the Queen of the Earth Sphere as a kind of figure head. Because of her skills as an orator she was able to become more than just a figure head in that her ideal of pacifism was shared by others of the Romefeller Foundation. After the death of Duke Dermail, Treize Khushrenada moved in and forced Queen Relena to give up her post as Queen and he moved in as her replacement, essentially becoming King of the Earth Sphere.
Treize led the military forces of the ESUN against the White Fang in the last battle of the war. During the battle Relena, with the help of Lady Une was able to show the people of Earth and the Colonies the futile nature of the conflict that had plagued the Earth sphere for many years. As a result, the Colonies renounced support of the White Fang group and called for peace between the Earth and the Colonies. Thus, a new era of peace and prosperity began with the end of the battle, and the ESUN became a democracy.
The ESUN continued to exist into AC 196 and cultivated a peaceful relationship between Earth and the Colonies. However, in December, AC 196, a small group known as the Mariemaia Army overthrew the ESUN with a sudden assault of Mobile Suits. The ESUN had no way to defend themselves because of its policy of total pacifism, (Which the conservative block disagreed with but could not do anything about it because the liberal block held significant power within the government) and the Gundam pilots (Except Wufei) had sent their Gundams on a collision course with the Sun. The pilots retrieved their Gundams and along with Preventer agents Zechs Merquise (Operating under the alias of Preventer Wind) and Lucrezia Noin were able to defeat the Mariemaia Army and restore the ESUN.
At the time the ESUN was created, the militaries of the nations of the world essentially became the military of the Earth Sphere. OZ once again formed the core of Earth's military as it had in the days of the Alliance. Under Queen Relena, a call was made for complete disarmament of the Earth, as well as the Colonies, but with the ascension of Treize Khushrenada, no disarmament was attempted. In fact, a large military force was assembled to deal with the White Fang which threatened the destruction of the Earth.
After the battle at the end of AC 195, the ESUN took a stance of total pacifism and dismantled all mobile suits against the conservative blocks wishes. Thus, the ESUN dissolved its main military force, but did reintain a small force known as the Preventers who still maintained mobile suits. The Preventers existed solely to diffuse potentially dangerous situations that could endanger the peace.