-people wanted to move because they wanted better lives. they faced dangerous journeys .
-they moved west because land was cheap or free and they thought they would do better business in the west .
-people who weren't making business or wasn't making any money, found the opportunities to move west to start everything all over again
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".
There are several ways in which the Chinese government reacted to demands for democratic reform, but most of them were violent suppressions of protests.
His teaching of the theory of evolution lead him to be arrested, as this broke a Tennessee state law. The AFT supported him, as he was willing to challenge government interference in with the science curriculum.
