Name the main reasons why people from Asia, Europe and Africa immigrated to the United States.

Early days: Europeans opened up new routes. A large number of Europeans went to America to look for gold, open up manors and slaughter Indians, resulting in a shortage of labor, so they transported black slaves from Africa to America through the triangular route. Later, the slave trade was banned, and we had to transport workers from densely populated Asia to America to fill the labor gap.

Later period: American economic law exhibition is rapid, and a large number of people go to the United States to look for business opportunities.

Africans were sold to the United States after being colonized, but some unyielding blacks escaped. As a result, millions of escaped blacks died. Asians were tricked into building railways in America, and many people died. However, those who survived and repaired the railway were the first to get green cards there. Europeans colonized everywhere at that time, and they came to America for their own benefit.