T he notion that America would be stronger if it was bigger preoccupied several 19th-century U.S. presidents, but none more than James K. Polk. America’s 11th chief executive believed that expansion was America’s God-given right and duty. He argued that broadening America’s borders would deliver access to natural resources and enhance national security—and that no nation, not even an ally, should get in the way of that. Beginning with the annexation of Texas, which voted to become the 28th