GEORGETOWN, Guyana — U.S. Secretary of State Marco Rubio vowed Thursday that the United States will respond forcefully if Venezuela attacks Guyana, amid an ongoing territorial dispute that includes massive oil and gas reserves. Rubio said it would be a “very bad day” for Venezuela should that happen. On the second leg of a three-nation tour of the Caribbean, Rubio was in Guyana as the Trump administration ramps up engagement in the Western Hemisphere to promote energy independence and curb