The historical development of capitalism drives inexorably (though not uniformly) toward the concentration of capital. This is expedited by increasing the scale of production, dominating markets, and improving technology. Concentrations of capital form monopolies that can exert proportional power (control) over the economic and political arrangements of the social formations they dominate.
When capital, ruled by its growth imperative, inevitably reaches limits to the accumulation of surplus value within the territory (nation, or social formation) it already controls, it must expand beyond its borders to conquer other areas. It uses the state(s) of its home base(s) to wage politics (up to and including war, the most extreme form of politics) on other social formations—to subjugate the ones it can, as well as to compete with others over how to carve up the world.
Though empires have existed before capitalism, imperialism is a specific stage of capitalism. Colonialism developed into imperialism with the internationalization of monopoly capital. More than simply an aggregate of national economies, imperialism is an integrated world system that is linked to a qualitatively increased socialization of production, and the complete partition and control of the world by the capitalist class.