The Greeks (well, Macedonians, if you prefer) built an empire that lasted 1,000 years, and stretched from the Mediterranean to Afghanistan. All during the lifetime of a warrior who died aged 32 (?). That was around 320BC.
So you have to go quite early in history, to find a culture that precedes "invade everything" imperialism (I doubt that there is any "beginning" to invade-everyone imperialism).
I’ll repeat it: the primary reasons which caused Japan to become “imperialist” are directly tied to Western imperialism. The Japanese emperor was mostly a political symbolic figurehead that united the Japanese islands, not a Roman emperor or British monarch intent on conquering the world.
This is all basic Japanese history.
Wikipedia article on Shogun[1] has a nice quick summary on pre-modern Shogunate system. It’s similar to British constitutional monarchy in a sense: the emperor was to delegate all power to the government and was to remain within instructions and supervisions of the government.