Amanto is the Japanese blanket name for the outer space aliens in the universe of Gintama. They and the organizations they formed have directly or indirectly influenced every character's lives in the series.
Background
Like what will later happen to the Earthlings, the Amanto had their lives changed when they discovered a revolutionary new power source called Altana, a planetary life force. When they found a way to convert it to energy, it could now be used to advance faster than light travel and upgrade the power of weaponry. This sparked a series of devastating wars over this valuable resource. Years later, the surviving planets decided to create an organization independent of their control to monitor Altana usage lead by a small group of wealthy Amanto, the future Tendoushuu. Thanks to having a monopoly on planets and their Altana, it is they who conquered worlds for their control and research. This also led to the Tendoushuu, and by extension the Amanto, to eye Earth for its Altana.
It is unknown when the Amanto invaded the other Earth countries, but they did confront the country with the biggest Altana hole (a place Altana naturally leaks from), Japan, which at the time was under an isolationist policy. Lead by the Inuisei, the Amanto ordered the bakufu to open its borders to the universe. Seeing how outmatched they were, the government concedes to their request. Some people refused to accept this and believed the government to be traitors to the country. Thus sparked a decade long civil war called the Joui Wars, that ironically allowed the Amanto to be more involved in their affairs. This included the Amanto sending their own armies and mercenaries to fight with the bakufu and the Tendoushuu ordering the government to conduct purges to wipe out the Joui. Thanks to these and the Amanto's advanced weaponry, the Joui were defeated. And for the next ten years until the manga/anime's present day, the Tendoushuu consolidate their power in the country and an influx of Amanto and their cultures entered the planet.
Thanks to the Amanto's influence, Japan, and implicitly, the rest of Earth, has become a mixture of (real life) mid 19th century, modern era and the future in culture and tech. An example of this mix is Edo: the center of the city stands the futuristic terminal where spaceships depart and arrive through wormholes leading to space. Surrounding it are buildings reminiscent of real life modern day Tokyo (Edo's present name) and further out from the terminal and cement buildings are mostly wooden buildings comparable to real life late Edo period.
Races
There are countless Amanto races shown that go unnamed but these are the known ones shown in Gintama:
This event is based on Japan's forced opening of its borders after two centuries and a half of isolationism by the more advanced country of the United States, followed by the European nations during the mid 1800's.
