The Social Contract tells us that individuals have their natural rights and loan it to government. But government cannot have powers loaned to them that the people themselves do not have. (We can have police enforce laws against armed robbery, because I as an individual can protect my property and self against armed robbery so I can “loan” that power to my government... On the contrary, I cannot steal money from my neighbor to help a person in need as that is theft; therefore, I cannot “loan” that power to the government either... i.e. redistribution of wealth is still theft).
If we take that view of legitimate government powers must originate from the individuals that loan those powers to the government, then how can I justify a non-defensive use of force against another country? If I, as an individual, cannot attack my neighbor then I cannot authorize my country to attack another country. However, if my neighbor attacks me I can defend myself and if a nation attacks the United States we can defend ourselves.
Most libertarians do not oppose national defense. Ron Paul voted FOR the operations in Afghanistan for example. However, that doesn’t justify “nation building” in Afghanistan or pre-emptive war in Iraq through the lens of the Social Contract.
If your neighbor threatened that he was going to rape and kill your wife and daughters, would you wait until he acted? We have a different understanding of the scope of a social contract. I do not agree that as an individual that I do not have the right of self-preservation that includes preemption. My social contract with the government is to act in my best interest, which would include preemptive actions. We no longer live in a world that allows us to be reactionary. The consequences of failure to act in a preemptive way is simply too grave. If you want to debate whether or nor a specific situation requires preemptive action, that is another matter.
Ron Paul did vote for that measure. However the resolution said nothing about 'building democracy' in Afghanistan.
Mission creep has started to turn that conflict into something that looks a hell of a lot like Vietnam.