They cannot fly, and the Laser and Heavy Laser-class BETA shoot down anything that does, rendering aircraft useless and making any aerial military operations much more difficult. The BETA vary greatly in size, with the smallest identified strain (Soldier-class) being around 3 meters tall, and the largest identified strains (Fort-class) at over 60 meters tall. By 2001, the world population was down to about 1 billion people. They quickly spread from their landing point, Kashgar, and proceeded to invade the Eurasian, Middle Eastern and Asian regions when they finally reached Japan in 1998, 36 million people were killed within a week.
They arrived on the Moon in 1967 and, after a brief conflict with humanity's space forces, invaded Earth in 1973.
Muv-Luv Unlimited and Alternative, as well as their associated side-stories, take place in alternate universes where the BETA were first spotted on Mars in 1958. BETA are featured in Muv-Luv Unlimited/ Alternative and their related spinoff works, where they are the primary antagonistic faction against humanity's forces.