In her provocative new book, Laurie Winn Carlson questions the larger aims of the famed Lewis and Clark expedition of 1804-1806 and sees it as part of a broad range of schemes to wrest the American West from the claims of established European powers. If American ships were already plying the waters off the Pacific Northwest coast, why, Ms. Carlson asks, was it necessary to send these two intrepid explorers overland-except as a demonstration of American reach, and perhaps as a ploy to tempt the Spanish to attack the expedition, thus provoking a war with Spain in Florida and the West. Ms....