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I agree that an isolationst USA would be dangerous, but I would also argue it is also dangerous to have a USA which involves itself in international affairs out of self interest rather than because of the responsibility which you say comes with power.
Every nation acts out of self-interest. Every one. That is what a government is for.
We deter aggression around the world when it is in our self-interest.
The African example is a good one. Why aren't we more involved there? Quite simply, because the last time we tried to help out (Somalia) the peacekeepers we sent were attacked. So we pulled out.
I agree that even though the US is powerful, we cannot solve every problem, resolve every conflict, feed every person.
Of course, I also believe that acts of charity (especially outside the US) should be done by private individuals or governments who are directly involved, not by the US government.