Hitler physically invaded Austria, although he also had some support from within the country, from people who believed all Austrians should be Germans. The invasion was later made to look like a voluntary incorporation with the helped of a staged referendum.
It would be "NAFTA", which stands for the "North American Free Trade Agreement" that is a treaty to eliminate all trade barriers between Canada, Mexico, and the United States, since each state believes this to be in thier best economic interest.
the natural barriers were the himalaya mountains
I think the answer is 1st Amendment of the Constitution. I think it is the answer because the amendment protects freedom of religion.