Cities were crushed by the falling spacecraft-but one ship didn't crash. It remained intact, and disgorged hordes of alien soldiers determined to fight to the death.   The abandoned vessels also contained a wealth of advanced technology. Led by David Levinson, the greatest minds of our world developed deadly new hybrid weapons. Bases were built on the Moon, Mars, and beyond.  A new generation of defenders had to be trained, for the invaders would return. In the headlong rush to prepare, however, not everyone would survive…