Earth Protection Alliance (EPA) :

The EPA was the mobile defence system of the Earth-Shield matrix, which consisted of a few hundred laser-turret satellites, space-mines and a small fleet of ships.

There had never been a threath from outer space so there was no real need for the Earth-Shield, it was born after a global law prohibited the use of laser-turret satellites. Until that time, most countries had a number of such satellites to protect themselves in case anyone would launch nukes at them. The laser-turrets would locate any incoming projectiles aimed at the country and destroy them.
Alot of money was invested in the turrets and no-one really wanted to get them back down to earth to have them dismantled, so they decided the point the lasers away from earth and into space to protect earth as a whole and that way they could all keep their satellites.
The Earth-Shield Corporation was created to oversee the use of the satellites and make sure none were used to destroy targets on earth. To convince the tax-paying citizens of the world the alliance launched a huge campain to convince everyone of how fragile earth would be without the Earth-Shield. Eventually the existence of the shield and alliance was accepted and more funds were made available to improve the shield : more laser-turret satellites than ever before were put into orbit around space and the few space stations between earth, mars and the moon were equipped with additional defensive systems and space mines.

Improvements to the space-shuttle fleet resulted in the creation of smaller space crafts which were not intended to transport people or goods between the space stations and earth, but which served as space-fighters. The first designs were very similar to the current jet-fighters and, in turn, resulted in the creation of the Vector shuttles. The increasing number of armed space craft led to several incidents and almost caused a global war, until the Earth-Shield Corporation created the Earth Protection Alliance, who would oversee the use of armed space craft and who also created the Space Academy to train deep-space pilots. The EPA would protect earth, not only from aliens but also from the countries who owned turret satellites or space fighters.

The number of pilots and space fighters increased and the space fighters themselves were improved and became impressive war machines, but they never fought any aliens, since no aliens ever attacked earth. The biggest threath to earth had always been mankind itself.