Scott
isn't happy with the writing on the History Channel:

They spend the whole season building up how the Japanese home islands are a fortress, and the Japanese will never surrender, and there's no way to take the Japanese home islands because they're invincible...and then they realize they totally can't have the Americans take the Japanese home islands so they have no way to wrap up the season.

So they invent a completely implausible superweapon that they've never mentioned until now. Apparently the Americans got some scientists together to invent it, only we never heard anything about it because it was "classified". In two years, the scientists manage to invent a weapon a thousand times more powerful than anything anyone's ever seen before - drawing from, of course, ancient mystical texts. Then they use the superweapon, blow up several Japanese cities easily, and the Japanese surrender. Convenient, isn't it?

...and then, in the entire rest of the show, over five or six different big wars, they never use the superweapon again. Seriously. They have this whole thing about a war in Vietnam that lasts decades and kills tens of thousands of people, and they never wonder if maybe they should consider using the frickin' unstoppable mystical superweapon that they won the last war with. At this point, you're starting to wonder if any of the show's writers have even watched the episodes the other writers made.