U.S. House Speaker Nancy Pelosi departed Taiwan after a whirlwind trip that raised military tensions over the island to their highest level in more than two decades, an escalation that is set to persist long after she returns home.

By the time Mrs. Pelosi (D., Calif.) wrapped up a day of meetings with Taiwan's president, senior lawmakers, executives and human-rights activists and left Taiwan on Wednesday, China's navy had already maneuvered into position for large live-fire drills that will encircle the island beginning Thursday.

Mrs. Pelosi's visit sought to reinforce what she said was America's ironclad commitment to preserving Taiwan's democracy. Yet the trip also brings Beijing's military activity into what Taipei claims as its territorial waters, according to Taipei's Defense Ministry, raising the prospect of greater pressure on an island that is the most sensitive flashpoint in U.S.-China relations.