A territory of 828,000 square miles of land spanning from the Mississippi River to the Rocky Mountains. This land was bought by the US from France in 1803 from 15 million.

A boundary set at 36°30′ north latitude by the Missouri Compromise in 1820. The line divided the Louisiana Purchase into free and slave territories, making states north of the 36th parallel free except for Missouri. This turned Missouri into a slave state and Maine into a free state.

The first battle of the American Civil War where Confederate forces fired on Fort Sumter, which was held by the Union. Eventually, Union forces surrendered. The conflict had no casualties, but led to more states seceeding and the mobilization of troops on both sides.

A three-day-long battle during the Civil War. It was won by the Union and is considered a turning point in the war, leading to the Union's ultimate victory in the long run. The Battle of Gettysburg was the bloodiest in American history, claiming the lives of 50,000 people.

Violent fights that occurred during a debate on the legality of slavery in the newly considered state of Kansas. There are 56 reported deaths from it, but the toll may be as high as 200. The legality of slavery in Kansas was an important issue due to it being a tipping point in the Senate, which was very divided over the issue of slavery. The Kansas-Nebraska Act led to the decision being made by settlers, and it was ultimately admitted as a free state. Bleeding Kansas was a state-level Civil War that was a prelude to the coming nationwide one.

Southern Democrats in North Carolina led a mob of 2,000 white men to overthrow the biracial government in Wilmington. It is considered to be a turning point in North Carolina's politics post-Reconstruction era, and was part of an era with more severe segregation and disenfranchisement of Black voters.

A gold spike that marked the completion of the first transcontinental railroad across the United States. It connected the Central Pacific Railroad from Sacramento and the Union Pacific Railroad from Omaha, Nebraska.

A 1911 fire in New York City that killed 146 workers was caused by a match igniting fabric scraps, and was worsened by locked exit doors that trapped workers inside and inadequate safety measures. It spurred a wave of activism that led to significant reforms in workplace safety, sanitation, and labor laws in the United States.

The September 11 attacks, colloquially known as 9/11, were four coordinated Islamist terrorist suicide attacks by al-Qaeda against the United States in 2001. Nineteen terrorists hijacked four commercial airliners, then flew one into each of the Twin Towers at the World Trade Center in New York City, and another into the Pentagon, the headquarters of the U.S. Department of Defense, in Arlington County, Virginia. The fourth plane crashed in a rural Pennsylvania field during a passenger revolt.