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The short answer to your Wall Street Journal question is "NO!"
The Posse Comitatus Act of 1878 has squat to do with the late arrival of federal troops or help in the post-Katrina devastation! No, no way! Nor does our historic and legal need to separate our "regular" federal (military) troops from our state (National Guard) "militia" have squat to do with the late arrival of federal (or state) assistance to the 100s of thousands left stranded without adequate shelter, food, medical assistance, and/or water!
Instead, blame where blame is due and on point: George Walker Bush.
Yes, this entire f*ck up was due to George Walker Bush's federal money cut-backs from FEMA and other federal (and state) programs! Hell, had not George Walker Bush totally emasculated the U.S. Army Corps of Engineers ability to follow-through with its program of shoring up the entire levy-system and other anti-flooding issues, then New Orleans would NOT be in this present chaotic and devastating situation. FEMA was cut-back more than 90% since 9-11, correct? The New Orleans anti-flooding program was cut-back more than what percentage u/ GWBush since 9-11?!
It's an outrage!
All because of neo-con and war-monger, George Walker Bush! And all since 9-11!
The Wall Street Journal and Mr/Ms Yochi J. Dreazen can wax and wane wringing "intellectual" legalese hands; however, it merely sells news print not reality.
The long answer is:
1.) There is a difference of legal authority granted in our U.S. constitution through our congress regarding what the "regular" military (federal troops) can do, as distinguished from what the state "militia" National Guard can do.
2.) The "regular" federal military has no police powers while on American soil (if not upon their own U.S. military bases in America), that means the federal military cannot arrest or enforce law, period.
3.) The state "militia" National Guard has police powers when authorized by a state governor, that means the National Guard may arrest and enforce laws.
And, there you have it. There are exceptions to these "rules" but none of the exceptions were in effect during Katrina or up to now, so none of the exceptions apply up to present.
As a great overview, here's what JURIST NEWS said about this very issue, earlier:
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=102x1745880.