The insistence of the United States on her trading rights was also important. Soon after the war began Britain, France, and their allies set up a naval blockade of Germany and Austria. Even food was contraband. The Wilson Administration complained bitterly that the blockade violated international law. U.S. firms took to using European neutrals, such as Sweden, as intermediaries. Surely, the Americans argued, international law protected the right of one neutral to trade with another. Britain and France responded by extending the blockade to include the Baltic neutrals. The situation was similar to the difficulties the United States experienced during the Napoleonic wars, which drove the United States into a quasi-war against France, and to war against Britain.
Ultimately, however, it was not the conventional surface vessels used by Britain and France to enforce its blockade that enraged American opinion, but rather submarines used by Germany. When the British (who provided most of the blockading ships) intercepted an American ship, the ship was escorted into a British port, the crew was well treated, and there was a chance of damage payments if it turned out that the interception was a mistake. The situation was very different when the Germans turned to submarine warfare. German submarines attacked without warning, and