Blockade precedent...
Blockade precedents are intersting.
In the American Civil War, the US Supreme Court ordered some seized materials that were being shiped to a neutral nation (Mexico) returned--even though they were being shipped directly from there across the river to Texas.
THIS precedent says that the USA can ship anything to neutral nations, where ever situated--such as Denmark, the Netherlands, or Italy.
Also, the British intercepted ships using the doctrine of continual voyage--if the ultimate destination was an enemy nation, than it was subject to seizure. In the American Civil War, they specifficly denounced the doctrine--bulky freighters could transfer any amount of military cargo to Bermuda, which was then transfered to blockade runners. HMMMM....Precedent favors free trade.
Under The Hague 1907, the USA would be within its rights to embargo arms evenly, or permit the sale evenly--and its trade with neutral nations should be inviolate. A blockade of beligerant powers is legal--including stopping neutral ships bound there IN A LEGAL BLOCKADE.
The government can't sell to any belligerant--but private companies can--even war materials of almost any sort. Warships may not be sold to warring powers--even by private companies within the neutral powers. (Bethlehem Steel's sub sales to Britian come cleanly under that provision.
The government supporting war loans also is a violation.
http://www.yale.edu/lawweb/avalon/avalon.htm
http://www.yale.edu/lawweb/avalon/lawofwar/lawwar.htm
The second link has all the relavant treaties.
And, of course, there's loots of ways to make the point about free trade without resorting to war. Some southern representatives or senators (I forget which) urged that we send "Ironclads" as escort. Certianly, an escorted convoy would make the point that we treat this seriously.
(And the usual problems of keeping a convoy together at night wouldn't be there--as neutral ships, they would sail brightly lit...)
Just a few thoughts