European immigrants left their home, their families, and their friends for a land that was vastly unknown and uncertain. Yet, they came for the sole purpose of more land, more money, and more freedom. They, like many other immigrants came for a chance at a better life. Many immigrants still come with the same mindset, they see the United States as a land of opportunity. The U.S. is full of jobs and good public education and that's why the U.S. seems to be such a hotspot for immigrants.
[Note: I don't know your grade level so I'm sorry if I made it seem too advanced or too "dumb" I hope this helps!]
The war began when the newly elected Holy Roman Emperor, Ferdinand II, tried to impose religious uniformity on his domains, forcing Roman Catholicism on its peoples, and the Protestant states banded together to revolt against him.
