In addition to trade with other countries and the interdependence of countries (think of where all the parts come from to build a car, for example), there's also been foreign aid. You'd need to research this to see when the US embarked on this path.
http://www.globalissues.org/TradeRelated/Debt/USAid.asp
Immigration IS very broad because it takes many forms, legal and illegal. And people come to the US from many parts of the world. A thorough study of all this would be an excellent element in your paper.
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Why is it not true that the US remained isolated from world events in the late nineteenth century? Describe some specific instances of American involvement beyond its own shores between 1865 and 1900.
I have two examples but i need a third. Immigration and the Open Door notes. I was also a little concerned that immigration was to broad an example. Any suggestions would be appreciated.
3 answers
You may also want to check out this chronology of events. Note that we bought Alaska from Russia and annexed Hawaii in this period.
http://www.scaruffi.com/politics/american.html
http://www.scaruffi.com/politics/american.html
Immigration from many countries in both Europe and Asia was an important part of that period.