The establishment of presidential term limits.
The government had agreed to stop funding Shinto, which those they made the treaty with hoped this would take power away from the Emperor. So Shinto became more of a culture thing instead of a religious thing.
Answer:
president polk supported abolitionism
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".
