Who came up with the separation of powers?​

your mum

Explanation:

she came up with the separation of powers to get your dad out of the house

The term "trias politica" or "separation of powers" was coined by Charles-Louis de Secondat, baron de La Brède et de Montesquieu, an 18th century French social and political philosopher.

Or you could put:

Separation of powers is a political doctrine originating in the writings of Charles de Secondat, Baron de Montesquieu in The Spirit of the Laws, in which he argued for a constitutional government with three separate branches, each of which would have defined abilities to check the powers of the others.

no because we were not ready for war