Preamble:
We
the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to
ourselves and
our Posterity, do ordain and establish this Constitution for the United States of America.
Article 1, Section 8.
To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions
http://usconstitution.com/constitution#articleiv
in·va·sion
[in-vey-zhuhn] Show IPA
noun
1.
an act or instance of invading or entering as an enemy, especially by an army.
2.
the entrance or advent of anything troublesome or harmful, as disease.
3.
entrance as if to take possession or overrun: the annual invasion of the resort by tourists.
4.
infringement by intrusion.
http://dictionary.reference.com/browse/invasion
Now, whether you agree with policy or not, my point is that the authority
is there.