These weapons were capable of striking the enemy's depths with precision, meaning that once a war broke out, enemy soldiers would be annihilated by missiles at their locations even before they had a chance to get out of bed. This is the terrifying aspect of long-range warfare.
When Earth's military forces originally invaded the Otherworld, they were actually exploiting the inequality in line-of-sight between the two sides. They intended to use their technological advantage through long-range warfare to deal with the Otherworld Allied Army, which was still engaged in line-of-sight battles.
However, the outcome was predictable. The magic of the Otherworld weakened Earth's military's long-range advantage and enhanced the line-of-sight advantage of the Otherworld armies. Thus, the advantages of both sides reached a basic balance at that time. With the Earth's military command in disarray, it was only natural that they were completely annihilated.