Hildebrand's election as Pope Gregory VII marked the beginning of one of the most consequential papacies of the Middle Ages. His insistence on papal supremacy over secular rulers in matters of clerical appointment ignited the Investiture Controversy, a struggle that would humble Emperor Henry IV at Canossa and reshape the balance of power between church and empire for centuries.

At Heiligerlee in the northern Netherlands, forces loyal to William of Orange's brother Louis of Nassau ambushed and routed a Spanish royalist army, killing its commander. The victory, though small in scale, is traditionally regarded as the opening battle of the Dutch Revolt against Habsburg Spain—a war that would last eighty years and eventually birth the independent Dutch Republic.

West Berlin university students, working in secret for months, dug a tunnel underneath the fortified Berlin Wall, successfully guiding dozens of East German refugees to the West before the passage was discovered by East German security forces. Such tunnels became a signature—and perilous—form of resistance to the Wall erected the previous year.

Two months before Apollo 11, astronauts aboard Apollo 10 undocked their lunar module and descended to within roughly 15.6 kilometers of the lunar surface, testing every procedure for landing short of actually touching down. The mission proved the hardware and techniques that would let Neil Armstrong and Buzz Aldrin land safely that July.

Disney opened River Country, a rustic-themed water park at its Fort Wilderness Resort near Orlando, offering swimmers a chlorinated lagoon designed to look like an old-fashioned swimming hole. Though modest by later water park standards, it launched Disney's decades-long expansion into aquatic entertainment and previewed the theme-park giant's ambitions beyond rides and rollercoasters.

After years of negotiation and periodic border wars, the Yemen Arab Republic (North) and the People's Democratic Republic of Yemen (South) formally unified into the Republic of Yemen, with Sana'a as capital. The merger ended a Cold War-era split between a conservative northern state and a Marxist southern one, though tensions over power-sharing would resurface in a brief civil war just four years later.

Masked gunmen burst into Oslo's Munch Museum during opening hours, threatened staff and visitors, and tore two of Edvard Munch's most famous works—'The Scream' and 'Madonna'—from the walls before fleeing in a getaway car. The brazen daylight heist stunned Norway and triggered a massive international manhunt for the missing masterpieces.