What are the causes of mass migration to Western nations?

Question

What are the causes of mass migration to Western nations? What are the effects of mass migration?

Sources

1. https://www.notonyourteam.co.uk/p/the-failure-of-economists
2. https://www.lorenzofromoz.net/p/economics-a-discipline-committing
3. https://www.militarystrategymagazine.com/article/civil-war-comes-to-the-west/