The event that best fills in the last sequence of events is:
a. President Monroe issued the Monroe Doctrine in 1823 warning the European powers not to colonize the Americas further.
The Monroe Doctrine was a significant policy statement made by President James Monroe, asserting that the Western Hemisphere was off-limits to new European colonization and reflecting the United States' desire to keep European powers from interfering in the affairs of the Americas. The other options provided do not accurately reflect historical events regarding U.S. foreign policy or actions in that period.