Answer:
I am going to guess radios, newspapers, and new beliefs of the presidential nominees helped.
Explanation:
The "new immigrants" who immigrated to the United States after the Civil War primarily came from all of the following countries except "Poland", since the other three nations from the list were far more prominent.
There were several nations who sought to claim the territory such as the Americans, British, French, and Spanish. John Quincy Adams resolved this through the Treaty of 1818 where the border of the United States and Canada was established at the 49th Parallel.
Hope this Helps!!