The Alpha Centurians (or 'Gray's as they are nicknamed by Humans) were a technologically advanced race who lived on the closest planet to Earth able to support life - Alpha Centuri Major, or Alpha Centuri. These aliens had visited Earth several times during its history, but thankfully, they never deemed it a threat. They discovered the League in 1888 PLF, and immideitly declared war on it. Were it not for the brave attack carried out on their homeworld by Spein and his forces, the Alpha Centurians might have dealt some significant damage to the unsuspecting League. The Alpha Centurians then joined the Galactic Council and continued to cause trouble for the League, and after the abolition of the Council they now provide much of the military forces for the Supreme United Confederate Alliance of Seperatists.
The Alpha Centurians themselves are short, bug-eyed, grey beings who fly around in disk-shaped starships. They have visited Earth several times but have never done any real damage, aside from one incident where Gray terrorists they attacked and almost destroyed London, for no apparent reason. They also visited several other pre-space age planets near to their star system until they discovered the League in 1888 PLF. At this time, humanity was still in an Industrial Age, but by the time the invasion force of Alpha Centurians was ready in 1988 PLF, humanity had rudimentary space technology. Spein utilized the help of a wizard to propel them to Alpha Centuri Major, where they attacked the fleet and caught the attention of the League, who soon stepped in and helped.
Generally, 'Grays' is a name given to any Alpha Centurians that live on Mars, and many of them did for a time.