On November 8, 1519 Cortés first entered Tenochtitlan, but by late December the Spanish had consolidated their tense, uneasy occupation of the city, effectively holding Moctezuma II hostage within his own palace. This bizarre standoff between a few hundred conquistadors and the heart of the Aztec Empire set the stage for the following year's catastrophic siege and collapse of Mexica civilization.

Having crossed a brutal Atlantic winter and signed the Mayflower Compact weeks earlier, the Pilgrims finally set foot at Plymouth on December 21 (Old Style December 11), beginning construction of the colony that would become a foundation myth of American identity. Half the settlers would die within months from disease and exposure.

After nearly 500 years of relative dormancy, Vesuvius erupted catastrophically in December 1631, sending lava flows and ash into villages around Naples and killing an estimated 3,000 to 6,000 people. The eruption reawakened Europe's memory of Pompeii's fate and prompted the first systematic scientific observations of the volcano's behavior.

Thomas Edison filed his patent application for the phonograph on December 21, 1877, just weeks after building the first working model, which recorded him reciting 'Mary Had a Little Lamb' onto tinfoil. The invention astonished contemporaries — sound, once spoken, had always vanished instantly; suddenly it could be captured and replayed, a feat many found almost supernatural.

In February 1897, a British 'Punitive Expedition' captured and torched Benin City, capital of the Kingdom of Benin, killing its defenders and exiling Oba Ovonramwen. Soldiers looted an estimated 3,000 to 5,000 bronze and ivory artworks, the famed Benin Bronzes, which were auctioned off in London to cover the expedition's costs and now sit in museums from Berlin to New York, at the center of an ongoing restitution debate.

At 12:51 PM Eastern, a Saturn V rocket lifted Frank Borman, Jim Lovell, and William Anders from Kennedy Space Center on the first crewed mission to leave low Earth orbit entirely. Three days later they would orbit the Moon ten times, and Anders would photograph 'Earthrise,' one of the most reproduced images in history, reframing how humanity saw its own planet.

Vietnamese troops crossed the border in force on December 21, 1978, after months of Khmer Rouge cross-border massacres against Vietnamese villages. The offensive, which captured Phnom Penh two weeks later, ended the Khmer Rouge's four-year reign of terror that had killed roughly 1.7 million Cambodians, but triggered a decade-long Vietnamese occupation and a bruising border war with China.

At 7:03 PM, a plastic explosive concealed in a cassette recorder detonated in the forward cargo hold of Pan Am Flight 103 en route from London to New York, tearing the Boeing 747 apart 31,000 feet above Lockerbie, Scotland. All 259 people aboard died, along with 11 residents killed by falling wreckage, making it the deadliest terrorist attack on British soil and a defining case in international law.

Leaders of eleven Soviet successor states met in Alma-Ata, Kazakhstan, on December 21, 1991, to sign the protocol expanding the Commonwealth of Independent States and confirming that the Soviet Union had ceased to exist. The agreement left Mikhail Gorbachev, still nominally the USSR's president, with no country left to govern; he would resign four days later.