The United States of America, commonly known as the United States, is a nation-state on the planet Earth. The American government is headed by a president.
Meeting the Autobots
The Autobots came to Earth prior to the Decepticons, who soon followed them, after intercepting a signal from Optimus Prime ordering for assistance on the small world. After escaping Shockwave's clutches on Cybertron, Arcee and Cliffjumper met with Optimus Prime on Earth and established a base of operations. Under the protection of the United States Military, the Autobots continued to battle the Decepticons and their leader, Megatron. Special Agent William Fowler was placed as the Autobots' liaison to the outside world and although, he wasn't excieted for what he considered a "Bot-sitting assignment," he obeyed and helped the Autobots in any way he could.
The War Continues
For three years, the Autobots had no contact with the Decepticons until the death of Cliffjumper, who was murdered by Starscream. Megatron soon returned and the military wanted to get involved, but Optimus didn't wish to put the lives of innocent humans at risk. The Autobots would continue to battle the Decepticons until the death of Megatron and the regeneration of their home planet.
After the death of Megatron, the Autobots left their human allies on Earth and returned to Cybertron, leaving Ratchet behind to help the United States track down the remaining Decepticons on the planet.