April 20, 1861: Colonel Robert E. Lee resigned from the United States army two days after he was offered command of the Union army and three days after his native state, Virginia, seceded from the Union. Two days later, Lee was appointed commander of Virginia’s forces with the rank of major general. He spent the next few months raising troops in Virginia, and in July was sent to western Virginia to advise Confederate commanders struggling to maintain control over the mountainous region. The next year, Lee assumed command of the Army of Northern Virginia.

April 20, 1898: President William McKinley asked Congress to declare war on Spain. Three years earlier, Cuba, located less than 100 miles south of the United States, attempted to overthrow Spanish colonial rule. The rebels received financial assistance from private U.S. interests and used America as a base of operations from which to attack. The Spanish military responded with brutal force; approximately 100,000 Cuban civilians died in wretched conditions within Spanish concentration camps between 1895 and 1898. McKinley originally tried to avoid an armed conflict with Spain, but the American media, led by newspaper baron Randolph Hearst, lambasted McKinley as weak and whipped up popular sentiment for a war to give Cubans their independence.
On February 17, 1898, the battleship USS Maine, moored in Havana’s harbor, sank after being rocked by two explosions; 252 men onboard were killed. Hawks in the media and within the government immediately blamed Spain, and President McKinley, abandoning his hopes for neutrality in the Cuban-Spanish conflict, bowed to Congressional calls for war. (It was later discovered that the explosion was caused by the spontaneous ignition of faulty ammunitions onboard the Maine.)

April 20, 1945: Allied bombers in Italy began a three-day attack on the bridges over the rivers Adige and Brenta to cut off German lines of retreat on the peninsula. Meanwhile, Adolf Hitler celebrated his 56th birthday as a Gestapo reign of terror resulted in the hanging of 20 Russian prisoners of war and 20 Jewish children: Of these, at least nine were under the age of 12. All of the victims had been taken from Auschwitz to Neuengamme, the place of execution.

April 20, 1980: The Castro regime announced that all Cubans wishing to emigrate to the U.S. were free to board boats at the port of Mariel west of Havana, launching the Mariel Boatlift. The first of 125,000 Cuban refugees from Mariel reached Florida the next day.

April 20, 1999: Two teenage gunmen killed 13 people in a shooting spree at Columbine High School in Littleton, Colorado. At about11:20 a.m., Dylan Klebold and Eric Harris began shooting students outside the school before moving inside to continue their rampage. By the time SWAT team officers finally entered the school at about 3:00 p.m., Klebold and Harris had killed 12 fellow students and a teacher, and had wounded another 23 people. Around noon, they turned their guns on themselves and committed suicide.

April 20, 2010: An explosion and fire aboard the Deepwater Horizon oil drilling rig in the Gulf of Mexico, approximately 50 miles off the Louisiana coast, killed 11 people and triggered the largest offshore oil spill in American history. The rig had been in the final phases of drilling an exploratory well for BP, the British oil giant. By the time the well was capped three months later, an estimated 4.9 million barrels (or around 206 million gallons) of crude oil had poured into the Gulf.