Question

During the 19th century, the United States ability to remain isolated from European affairs was largely due to its

Answer 1

I think that the best answer is the geographic location: many other countries (Sweden for example) have tried being neutral at different times but did not manage to: but the fact that the US has only two boundaries, and that both other countries also have a history of fighting from independence from a European power,  makes it possible that unless US decideds to get involved in international politics, it can be "left alone".