April 13 in Military History

After 34 hours of Confederate bombardment, Major Anderson surrendered Fort Sumter. No soldiers died during the bombardment itself, though two Union soldiers were killed during the evacuation salute, the first military casualties of the Civil War.

British Brigadier General Dyer ordered troops to fire on thousands of unarmed Indians at Jallianwala Bagh in Amritsar. Soldiers fired 1,650 rounds, killing at least 379 people. The massacre became a turning point in India's independence movement.

Germany revealed mass graves of 4,400 Polish officers executed by the Soviet NKVD in 1940. The revelation exposed one of WWII's greatest war crimes and severed Soviet-Polish diplomatic relations.

Soviet forces completed the capture of Vienna after fierce street-by-street fighting during the Vienna Offensive, which involved the 2nd and 3rd Ukrainian Fronts against German Army Group South.

CIA Director Allen Dulles formally authorized Project MKULTRA, a multi-program effort to research chemical, biological, and radiological means of behavioral modification for intelligence applications.

A routine cryogenic stir caused Apollo 13 oxygen tank number two to rupture, venting the service module's oxygen supply and forcing the crew to abandon the command module for the lunar module as a lifeboat.

North Vietnamese forces assaulted An Loc, a provincial capital 60 miles north of Saigon, as part of the Easter Offensive. The 66-day siege pitted 35,000 NVA troops against a single ARVN division supported by massive U.S. air power, including B-52 strikes.

An SR-71 from Detachment 4 at RAF Mildenhall flew a reconnaissance mission over Libyan air defenses that had been rebuilt following the 1986 Operation El Dorado Canyon strikes. The sortie photographed new SA-5 batteries and Soviet advisors.

U.S. Marines of Task Force Tripoli secured Tikrit, Saddam Hussein's hometown and the last major Iraqi city holding out against coalition forces. The capture effectively ended the organized phase of the Iraq invasion.

The U.S. military dropped a GBU-43/B Massive Ordnance Air Blast bomb, the largest non-nuclear weapon in the American arsenal at 21,600 pounds, on an ISIS tunnel complex in Nangarhar Province, Afghanistan. It was the first combat use of the weapon.