Multiple empires can co-exist simultaneously. The US actually already had colonies since 1898 so it would already qualify as an empire at that point. You can ev

Multiple empires can co-exist simultaneously. The US actually already had colonies since 1898 so it would already qualify as an empire at that point. You can even make a case that the US started in empire in 1848 when it annexed Mexican land. The 1970's is way too late. Even my initial guess of post-WWII is late.