Traditional African music combined with new Western instruments like drum kits, guitars, etc. This produced new styles of music (Blues & Jazz among others). These styles along with the instruments then made their way back to Africa, which in turn affected and changed African music.
It's the basics of cultural exchange: in any exchange both sides are affected.
Now an artform created out of a combination of traditional African elements and African descendents experiences in North America has branched out into the world and affected all cultures that have come into contact with it.
Hip-hop will always be American, but it's affects are global.