Immigration to the US is done by people of different countries in the hope of better living conditions.
What kind of countries have the most immigrants?
Poland has been the largest of the immigrants to the US
Is there a reason why Poland has so many immigrants?
It seems it was because of World War 1 and 2
That's understandable. Poland was a large battlefield for a lot of the world wars and got invaded many times.
European interwar economy
The European interwar economy (the period between the First and Second World War, also known as the interbellum) began when the countries in Western Europe were struggling to recover from the devastation caused by the First World War, while also dealing with economic depression and the rise of fascism. Economic prosperity in the United States during the first half of the period was brought to an end with the onset of the Great Depression in 1929. 
In 1919 a new democracy was formed in a German town known as
urban districts.
Geography Metropolitan regions.
Germany officially has eleven metropolitan regions. In 2005, Germany had 82 cities with more than 100,000 inhabitants.
Immigration.
Over ten million people living in Germany today were born outside of Germany; equivalent to 12% of the German population. Most immigrants come from other European countries, particularly from Turkey, Russia, Poland and Italy. Germany is the second-most popular destination for immigrants in the world after the United States. The German Government has been keen to encourage immigration