Short answers, pulled from the story.
The Empire of Liberty is a foreign policy concept first developed by Thomas Jefferson to describe the United States' responsibility to spread freedom across the world. Jefferson first used the phrase in 1780, while the American Revolution was still being fought. He envisioned it encompassing westward expansion across North America and active intervention abroad.
Jefferson first wrote the phrase "Empire of Liberty" in 1780. He used it in the context of securing a barrier against British Canada and expanding the territory available for republican self-government.
Major exponents of the Empire of Liberty concept include James Monroe (Monroe Doctrine), Andrew Jackson, James K. Polk (Manifest Destiny), Abraham Lincoln (Gettysburg Address), Theodore Roosevelt (Roosevelt Corollary), Woodrow Wilson (Wilsonianism), Franklin D. Roosevelt, Harry Truman (Truman Doctrine), Ronald Reagan (Reagan Doctrine), Bill Clinton, and George W. Bush (Bush Doctrine).
The Monroe Doctrine, introduced in 1823, was the first major institutional translation of Jefferson's idea into standing U.S. foreign policy. It declared that European efforts to colonize or interfere with states in the Americas would be treated as acts of aggression. Many Latin Americans viewed it not as a defense of liberty but as justification for U.S. imperialistic relations with Latin America.
The Empire of Liberty concept provided motivation for the Spanish-American War (1898), U.S. entry into World War I (1917-18), the later part of World War II (1941-1945), the Cold War (1947-1991), and the war on terror (2001-present).
Richard Drinnon argued that Jefferson's stated ideals of an Empire of Liberty stood in stark contrast to his actual policies. Drinnon documented that Jefferson acquired roughly a hundred million acres from Native Americans through treaties "shot through with fraud, bribery, and intimidation," and that Jefferson himself initiated the Indian removal policy.