14: The 14th Amendment to the U.S. Constitution, ratified in 1868, granted citizenship to all persons born or naturalized in the United States—including formerly enslaved people—and guaranteed all citizens “equal protection of the laws.”
15: “The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude.”
16: The 16th Amendment to the U.S. Constitution was ratified in 1913 and allows Congress to levy a tax on income from any source without apportioning it among the states and without regard to the census.
Conflict with Mexico began when the United States annexed Texas as a state in 1845. Mexico claimed that the new border between Texas and Mexico was the Nueces River, while the United States contested the border was the Rio Grande.
Without a doubt, the United States has never really lived up to the ideals of the Declaration. Historians, myself included, are quick to point out that Thomas Jefferson enslaved several hundred human beings when he crafted the Declaration.