Metagross have never been used in large numbers for war, for catching one Beldum is hard enough; catching four is a feat which alone proves the greatness of their trainer. But when they have been seen on battlefields, they have single-handedly changed the outcome of wars in every period of history. Metagross are immune to Vileplume gas, swords, and bullets alike, there are few thing a modern soldier, let alone an ancient one, can do when faced with these levitating tanks. Explosives often fail to wound these pokemon, and even fire types struggle with their superb defenses and ability to smash the ground and spark earthquakes. Offensively, a single Metagross can clear a trench or destroy fighter jets by using its magnetism to call down meteors from the sky, or simply strike anywhere with fast-moving punches of their own; their punches called "bullet punches" as much for their speed as for the bullet-shaped chunks of metal they inevitably leave embedded in their victims.
Yet Metagross have the brains of supercomputers, and receives enormous amounts of data even while waging war. It is not unknown for Metagross to run away from combat or float above the battlefield, having come to the age-old conclusion that no matter how decisively they can win a battle, there are no victors in war. They often follow up this defection with assaults on the officer corps and politicians of both sides of the war, or printing out the results of their calculations which make very clear to the general public that both sides lose far more than they gain.
It is the Metagross as much as anyone who are responsible for ending the age of war.