One of the biggest reasons mankind has fled another country is to seek refuge and safety in another. One of the earliest modern reasons is to flee religious persecution. The Pilgrims left England by boat to flee to America to escpae the persecution they were experiencing from the Church of England. They fled to a country that had barely been explored and hardly anyone knew anything about. The Mormons fled by handcart to what eventually become known as Utah, but what was Mexico at the time.
Another reason people flee is for safety in a country that has been torn apart by war. Many people in the Middle East have been fleeing for decades to escape their war torn countries, where women are treated as animals, and children's safety is at risk daily.