In one of my LinkedIn groups today, an attorney posted the question of whether there were any cases explaining what is the "public enemy" as it is one of the few enumerated defenses to a motor carrier's liability under the Carmack Amendment. No one found any cases explaining what it is in that context, so I turned to US COGSA and anything else I could get my hands on. Below is my response.
The best I can synthesize is that the "public enemy" must be one that is an enemy of the sovereign itself, not of the owner of the goods.
I did not find any Carmack cases on the subject. For an analogous view, I looked at the admiralty side, and the Carriage of Goods By Sea Act (TITLE 46, App. Ch. 28 § 1304(f), wherein "Acts of Public Enemies" is an enumerated defense.
See International Harvester Co. v. TFL Jefferson, 695 F. Supp. 735, 739 (SDNY 1988), ("the common law provided an exception to a carrier's absolute liability for the safe arrival of goods when the loss was caused by "the public enemy" or "the King's enemy" and the carrier was not at fault. See G. Gilmore & C. Black, supra, § 3-22 at 139-40; 1 Carver's, supra para. 20.").
Referring to my own treatisies, "[A]n act performed in the prosecution of war might be the act of an enemy of the carrier's country, though the term might be equally well applied to the actions of military or naval forces of a nation at war with some country unconnected with either the carrier or the goods." G. Gilmore & C. Black, supra, § 3-34 at 164. And, in Schoenbaum, Admiralty and Maritime Law Hornbook, Carriage of Goods, § 8-29 at 589 (2d Ed. 1994), "Act of war need not be declared war in the international sense. Act of public enemies refers to enemies of the sovereign of the shipowner. Russel v. Niemann,  17 C.B. (N.S. 163). Any bellicose act including terrorism would appear to be covered. Scrutton, Charter Parties, 222.
And back in ancient history--Coggs v Bernard (1703) 2 Ld.Raym 909, by Lord Holt:
"As to the fifth sort of bailment, viz., a delivery to carry or otherwise manage, for a reward to be paid to the bailee, those cases are of two sorts; either a delivery to one that exercises a public employment, or a delivery to a private person. First, if it be to a person of the first sort, and he is to have a reward, he is bound to answer for the goods at all events. And this is the case of the common carrier, common hoyman, master of a ship, &c.; which case of master of a ship was first adjudged, 26 Car. 2, in the case of Mors v. Slew, Raym. 220. 1 Vent, 190, 233. The law charges this person thus entrusted to carry goods, against all events, but acts of God, and of the enemies of the king. For though the force be never so great, as if an irresistible multitude of people should rob him, nevertheless he is chargeable. And this is a politic establishment, contrived by the policy of the law, for the safety of all persons, the necessity of whose affairs oblige them to trust these sort of persons, that they may be safe in their ways of dealing; for else these carriers might have an opportunity of undoing all persons that had any dealings with them, by combining with thieves, &c, and yet doing it in such a clandestine manner as would not be possible to be discovered. And this is the reason the law is founded upon in that point." See <http://www.lawandsea.net/List_of_Cases/C/Coggs_v_Bernard_1703_2_LdRaym_909.html>.
More recently, in New York Marine & Gen. Ins. Co. v. M/V "RAUTEN", 1999 U.S. Dist. LEXIS 1513 (S.D.N.Y. Feb. 11, 1999), a COGSA case wherein CSAV moved for summary judgment based on the "acts of public enemies" defense, Judge Chin denied the motion in part because there was not a clear definition of what are acts of piracy or public enemy (citing to David Crystal).
As an example, in a post 9-11 Carmack world, I would argue that a truck load of ammonium nitrate moving interstate hijacked by a terrorist cell connected with al Queda would rise above the level of mere hijacker and would be an act of a public enemy, assuming of course that the intention that the goods would be used in carrying out an attack on the government and/or the public.
I am all ears for any input on this very interesting topic.