It's kind of explained in the first trailer
Postwar Japan had lost everything
From Zero to Minus
So if Godzilla Minus One is a modern technology / VFX take on the original Godzilla movie then the movie takes place almost immediately after the war.
Japan has been fire bombed and atomic bombed (!) to almost complete destruction (zero). So under those already awful circumstances, this previously unimaginable creature wreaks havoc on what's left of Japan and thus Japan goes from zero to minus one.
Well, at least that's my take