A reader:

I believe the Coast Guard is part of the United States armed services, not a police force. Gibbs, of course, as a member in good standing of the September 10thers, considers attacks on the mainland by foreign aggressors to be law enforcement problems, not acts of war.

UPDATE: A few readers have emailed to clarify:

The military is not allowed by law to preform functions that the Coast Guard does. The Coast Guard is a very special animal .
http://www.fas.org/sgp/crs/homesec/RS21125.pdf
DHS specifies five homeland security missions for the Coast Guard :
1 : Ports, waterways and coastal security. (Police Patrols)
2 : Drug interdiction. (Boarder patrol.)
3 : Migrant Interdiction (Boarder patrol.)
4 : Defense Readiness.
5 : Other law enforcement.
It also regulates the waterways and small boating…
The Coast Guard is not a real Military force based on function (They may think so.) but if you look at what the Coast Guard does it is really more like a national police force for nationally controlled waterways and coastal waters.