The United Earth Starfleet, more commonly known as Starfleet, was the military and exploratory arm of United Earth from the 2130s through 2161, when it was succeeded by the Federation Starfleet.
Under auspices of United Earth's Starfleet humanity made steady progress in developing warp speed with the Vulcans supervising and assisting. In the 2140s Starfleet launched the Franklin, which was the first Earth vessel to reach warp four, then followed by the Enterprise in 2151, which was the first Earth vessel to reach warp five.
Enterprise spearheaded Starfleet's exploration of the Alpha Quadrant, making first contact with a number of species, including the Tellarites, Axanar, and Andorians. Due to the efforts of the Enterprise a number of the Alpha Quadrant races put aside their differenced and came together to form the Collation of Planets in 2154.
From 2156 to 2160 the expansionist Romulans fought a war against United Earth and her allies. Starfleet was at the forefront in defending Earth and her allies in that conflict, and ultimately were able to force the Romulans back behind their borders after the Battle of Charon.
In 2161 Earth, Andoria, Vulcan, Tellar, and Alpha Centauri founded the United Federation of Planets. The United Earth Starfleet was then absorbed into the new, larger Federation Starfleet, contributing its personnel, ships, and material to the combined fleet. United Earth Starfleet's ship designs were seen as the most versatile of the founding states, and their ship designs heavily influenced the Federation Starfleet's ship designs for centuries to come.