In an apparent flexing of newfound muscles, the European Union is picking another trade fight with the United States, threatening to file a World Trade Organization complaint against the U.S. over fees on cargo ships at U.S. ports, increasing tensions between the two trading giants. In a letter released last Tuesday, European Commission vice president Leon Brittan said harbor fees proposed by the Clinton administration, like those they would replace, constituted an unfair tax on European shipping lines, container vessels and their cargoes. "This discriminatory application of a fee that is not justified in the first place simply cannot continue," Brittan told U.S. Trade Representative Charlene Barshefsky. If a settlement is not reached by Jan.