The economic sanctions of United States, due to the military aggressions carried out by Japan forces along Asia and Pacific sea, lead the imperium to start the offensive against US. Japan had a clear advantage before the battle of Midway, where Imperial Japan Navy lost a significant ammount of military resources, including several air carriers. After that, imperial forces threatened US, enforcing them to fight along heavily fortified islands.
Japan is great when it comes to hit and run tactics. Like the soviets, they are very zerg-like in nature and have an interesting balance system to show this. Like the soviets, their tanks are cheaper than average, but the difference being that they are significantly cheaper and trade almost everything a tank can bring to the field in favor of a bigger gun.
Japan infantry may not be the cheapest, but they are most relient on indirect firepower and outside support, making them the only actual infantry that is not quite fit for fair combat.
Although they have the cheapest armor in the game, it's also the weakest. Most of the tanks a Japan player can field don't qualify for medium class.
Japan infantry are also the least useful in direct combat, as they heavily rely on knee mortars and other possible support weapons.