Flat Out War, from Oversoul Games is touted as a set of universal rules for 25mm miniatures. It’s for “experienced gamers,” since it does not explain or illustrate a lot of common terminology.
Politics By Other Means is a miniatures matrix game designed to cover all eras.
Soldiers In Time is a set of free wargames rules that let you conduct skirmishes between soldiers of different eras.
I want to play the guys with the machine guns against the stone throwers.
OPUS is a one page, universal role playing system.
Universal Soldier was first published by RAFM in 1977. It is a universal game both in the periods it permits one to play, and in the sliding scale that lets players go from skirmish to large scale battles.