Depending on the time, it could be the Phoenicians ( I think this is the best answer; this is what they're known for best) or later in time, the Romans (but they had colonies also elsewhere).
What got people to move to the west were the great plains were, is because of the gold rush. The president showed a giant crowd of people a jar of water, sand, and gold nuggets. He told them this is what they would find if they moved to the west. And of course everyone went searching for gold.
