When Microsoft Corp. won a patent suit against Motorola Mobility Inc. at the International Trade Commission last year, it looked like a decisive victory. The ITC issued an exclusion order directing U.S. Customs and Border Protection to bar the importation of all infringing Android phones made by Motorola — a seemingly brutal blow.

In reality, it turned out to be just the first round of the fight. Microsoft this month sued the federal government in U.S. District Court for the District of Columbia to enforce the judgment, alleging that Customs violated the Administrative Procedure Act by continuing to allow imports of infringing phones.