Posse comitatus does not apply to the use of our armed forces in combating an invasion and that is what we have on our southern border.
Re: Posse Comitatus Act
House Permits Pentagon to Assign U.S. Military Along Mexican Border
Friday, May 12, 2006 - Associated Press
WASHINGTON — The Pentagon is looking at ways the military can help provide more security along the U.S. southern border, defense officials said Thursday, once again drawing the nation's armed forces into a politically sensitive domestic role.
On Capitol Hill on Thursday, the House voted 252-171 to allow Defense Secretary Donald H. Rumsfeld to assign military personnel under certain circumstances to help the Homeland Security Department perform border security. The House added the provision to a larger military measure.
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Under the Civil War-era Posse Comitatus Act, federal troops are prohibited from performing law enforcement actions, such as making arrests, seizing property or searching people. In extreme cases, however, the president can invoke the Insurrection Act, also from the Civil War, which allows him to use active-duty or National Guard troops for law enforcement.