CARACAS, VENEZUELA - As Venezuelan military officers barked orders, teenagers, grandfathers and housewives paraded across a flood-lit gravel lot, saluting and chanting patriotic slogans. Dressed in white T-shirts and black baseball caps, volunteers in the newly formed network of "popular defense units" vowed to protect President Hugo Chavez's leftist government from an American invasion. "We must be prepared to defend the homeland," declared a sweaty Thai Grimon, 34, as she cradled her 2-year-old son in her arms following 90 minutes of drills. U.S. officials scoff at the notion of a military assault on oil-rich Venezuela even while ratcheting up their...