Science:
- Mariner 6 begins transmitting far-encounter photos of Mars
Military History:
Vietnam War - On July 23rd, 1969 three military operations were in progress: Operation Distant Trumpet (involved 5th Bat., Royal Australian Regt); Operation Strangle (involved 1st Inf. Division); and Operation Idaho Canyon (involving 3rd Bat. 3rd Marines & 101st Airborne). There were many other operations that were still in progress, but these three were the closest to 23rd of July, 1969 start date.
In WWII, on July 23, 1942 - Rostov falls to German forces; 240,000 Red Army soldiers are taken prisoner.