1. The 1920 Missouri Democratic State Convention was held on August 18, where candidate James M. Pendergast was nominated for the U.S. Senate seat. Pendergast was associated with the political machine run by his brother, Tom Pendergast, which was a major force in Missouri politics during the early 20th century.

2. On August 18, 1934, the Clyde Barrow Gang (as in Bonnie and Clyde) was ambushed and killed by law enforcement officers in Bienville Parish, Louisiana. The gang had a history of robbing banks and killing law enforcement officers in Missouri and other states.

3. The Missouri State Fair, which takes place annually in Sedalia, officially opens on August 18. The fair features agricultural exhibits, carnival rides, live music, and a variety of competitions, including a livestock show.

4. August 18, 1977 marks the date of the "Viking 2" spacecraft's landing on Mars. The spacecraft was designed to investigate the surface of the planet and search for signs of life.

5. On August 18, 1999, the Missouri Supreme Court ruled that a ban on same-sex marriage was unconstitutional. The ruling paved the way for the legalization of same-sex marriage in the state in 2014.