You want to talk about lazy writing? You want to talk about deus ex machina? The whole thing gets suddenly cut short by a new mad scientist invention that is orders of magnitude bigger than anything used up to that point. Why even bother with any fighting to begin with? Just pull a crazy ass big bomb out of your butt and obliterate the other side.
They more or less ended the European part of it with an exciting large scale invasion and takeover, then decided to abruptly end the Pacific part of it with some bad science fiction.