I think it's more accurate to say there weren't many expansionist tyrants whilst the US was looking interested in world policing. The Soviet Union had to be very careful about what they did in Europe despite having their own nuclear umbrella, Saddam could tyrant all he wanted until he annexed a neighbouring state the US felt vaguely positively disposed towards, and whilst you could fight petty border wars or maybe fund a coup against a neighbour somewhere less strategic you didn't have the option of doing what 1939 German and Japan and Russia did or even what 2020s Russia is trying to do and China is probably thinking of.