From a reader:
The monsters of Japanese monster movies are metaphors for various powers that might threaten Japan. In the early 1950s when the atom bomb was still fresh in the minds of the Japanese, Godzilla was a nuclear monster that rose up from the sea to destroy Japanese cities (Godzilla = USA) This Godzilla was unambigously bad. The film was heavily edited for its American release, partly (so I’ve heard) to disguise that fact. You can chart the rise and fall of Japanese anti-Americanism by examining whether Godzilla is a good guy or bad in each movie in which he appears. After WWII, as America was increasingly seen as a protector of Japan from China or the Soviets, Godzilla was likewise seen protecting Japan from the monster du jour. Eventually the Japanese became more self reliant (Enter Mothra, who is Japan itself) and several movies where Godzilla and Mothra stand together to fight invaders, as strong allies tend to do. More recently we have a cultural backlash, and Godzilla, when he appears, has taken on more menace. Mothra is still always a good guy.
The WWII theme still plays well in monster movies. Read this summary of a recent one :