Why Not Mercenaries?
Waging war is a very serious matter. Commendably, U. S. Presidents and Congress are very reluctant to authorize the use of the Armed Forces of the United States. And yet, the U.S. has enemies, and often the national interest requires the use of military force short of a formal declaration of war. When diplomacy is exhausted, and military action is called for, Congress and the President need more than the two binary options of, either a formal declaration of war, or inaction. Currently, covert actions, or executive orders of questionable constitutionality, are the Presidential alternatives. But neither may be adequate in some cases. So, why not bring back letters of marque? The Founding Fathers wrote them into the Constitution for good reason.

