On June 18, the following notable historical events related to Missouri have been recorded:

• 1812: The War of 1812 began with the U.S. declaration of war against Britain. Missouri, then a territory, played a role as a frontier region, with conflicts involving Native American tribes allied with the British, particularly in the St. Louis area.
• 1874: The St. Louis Browns, a precursor to the modern St. Louis Cardinals, played one of their earliest professional baseball games, marking the city's growing prominence in American sports.
• 1942: During World War II, the Weldon Spring Ordnance Works in St. Charles County, Missouri, began operations. This massive facility produced explosives, contributing significantly to the war effort.