Rand on Government

I used to be a pretty strong advocate of Ayn Rand's Objectivist philosophy, but there is one thing I still cannot wrap my head around.  As far as I can remember, she based her political philosophy, rightly so, on an axiom of non-aggression -- no one should initiate the use of force against another.  But how then did she arrive at her advocacy for a government with a monopolistic power to use physical force?  What happens if someone refuses to participate in the government system?  Are they to be forced?  
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