From ADV:
In 1945, at the close of the Pacific War, an invasion beset the planet Earth. Droves of Phantom Beasts - aliens from another world - landed and waged war against man. Humanity was forced to unite and fight this common threat, but years later, in present times, the death toll has depleted the military forces, leaving Japan with no choice: They must enlist the help of teenagers. Atsushi Hayami is among those who, rather than lead a normal high-school life, must take up arms to ensure the survival of mankind.