A) Louisiana Purchase Treaty of 1803 INCORRECT The Louisiana Purchase was a treaty with France that doubled the size of the United States by acquiring territory from the Mississippi River to the Rocky Mountains. It was a domestic expansion measure, not a foreign policy doctrine. The excerpt likely refers to a policy opposing European intervention in the Americas, which the Louisiana Purchase does not address. The purchase was driven by the need for control of the Mississippi River and New Orleans, not by a stance against European colonization.
B) Adams-Onis Treaty of 1821 INCORRECT The Adams-Onis Treaty, also known as the Transcontinental Treaty, was an agreement between the United States and Spain that ceded Florida to the U.S. and defined the boundary between the U.S. and New Spain (Mexico). It was a bilateral treaty resolving territorial disputes, not a unilateral policy statement about European involvement in the Americas. The treaty was signed in 1819 and ratified in 1821, but it did not establish a broad foreign policy principle like the one described in the excerpt.
C) Monroe Doctrine of 1823 CORRECT The Monroe Doctrine is the correct answer because it is the landmark executive action that established the policy of opposing European colonialism in the Americas. In his 1823 address to Congress, President Monroe declared that the American continents were no longer subjects for future colonization by any European power. He also stated that any attempt by European nations to extend their system to any portion of this hemisphere would be considered dangerous to our peace and safety. This doctrine was a bold assertion of U.S. influence in the Western Hemisphere and has had a lasting impact on U.S. foreign policy.
D) Treaty of Guadalupe-Hidalgo of 1848 INCORRECT The Treaty of Guadalupe-Hidalgo ended the Mexican-American War and ceded a large portion of Mexico's northern territory to the United States, including present-day California, Nevada, Utah, and parts of Arizona, New Mexico, Colorado, and Wyoming. This treaty was a territorial acquisition, not a foreign policy doctrine. It occurred later than the Monroe Doctrine and dealt with a specific conflict, not a general policy against European intervention.
