It is usually viewed as the turning point
when the US became a world power in the common sense of the term; its
economy had already brought it there but having formal colonies made it
one of the big boys on the world stage. It’s earlier colonies were not
thought of as colonies per se, so having its population move into the
western US could be thought of as colonization (certainly from the view
of the locals) but the Spanish-American war established it clearly since
there was nothing natural about the US expanding into the far Pacific
as an occupier.
I hope this helps.
