In the news, currently, is exploration into the idea of using U.S, troops to guard our international borders. It seems that both the defense department, and governors of the border states are looking for authority and funding for such a venture.
The Posse Comitatus act of 1878 has been mentioned as a possible barrier to such a plan. After reading the act itself, then the mission of our armed forces, it seems obvious to anyone that doesn't have his or her head buried in the sand, that this is constitutionally the primary function of our military.
Our Army, etc. is commissioned for the purpose of protecting us from all enemies, foreign and domestic. Perhaps the most basic of this principle is the protection of our borders from invasion. Invasion doesn't have to be battalions or divisions, it can be 20 or 30 people walking across the border at night. Some to find work, others, I'm sure for more serious purposes.
If an uneducated peasant from Mexico can successfully infiltrate our borders, it should go without saying that terrorists can do even better.
This blogger believes that not only is using the military legal, but vital to our security.