Summary
The Cranassians are the villainous force from the Raiden series. These are alien invaders who want to conquer our world, starting an invasion in the year 2090. The World Alliance Military captures one of their vessels, and based on it they create the Raiden series of fighters, in order to counter the Cranassian invasion. The World Alliance Military has been successful in their fight against the Cranassians, having defeated them four consecutive times, though the Cranassians don't show signs of ever giving up.
The Cranassians headquarters is located on a large space station, surrounded by an asteroid belt.
Their forces are comprised of various military machines and aircraft, all of which are controlled by a red crystal(called Energen Crystal) which also serves as a power source. The crystals themselves are apparently sentient, since whenever their vessels are destroyed, the crystal will leave the machine and fly away.
The Truth
During the Earth-Cranassian Wars(also referred to as Crystal Wars), Earth's World Alliance Military forces managed to capture some Cranassian prisoners of war. One of prisoners tells the story:
Cranassians are humanoid alien race hailing from Planet Cranassia. Since 19th Century Planet Cranassia had been involved in constant wars, conflicts and strifes. Then came the 5 Great Global Wars that eventually destroyed the Planet Cranassia. Having destroyed each others' nations and their planet, Cranassians of different warring states are forced to forge temporarily alliance to seek a new home.
Thus began their long, arduous journey through space and time.
Finally in 2090 A.D. (event of the first Raiden), Cranassians reached The Solar System and discovered a very habitable 'Green Planet' called Earth. Cranassians, exhausted of resources and strengths after long and tortuous journey through space, decided that Earth would be theirs for the taking.
Furthermore, the POW confessed that during the 5 Great Global Wars of Cranassia existed a squadron of fighters with extremely high-performance. These extremely powerful fighters turned out to be distant protoypes of Supersonic Attack Fighter, which would later become World Alliance Military's Raiden Mk-II and Mk-II Beta.
These prototype fighters are called Raiden Fighters.
Raiden Fighters, Raiden Fighters 2: Operation Helldive and Raiden Fighters Jet depicts the first three Great Global Wars of Cranassia out of 5.