The Battle of Boonville took place on June 17, 1861 around Boonville Missouri. Boonville is just about half way between Kansas City and St. Louis. In fact this was the first of what would turn out to be four battles fought around Boonville. The last was in 1864.
The importance of this little know battle is that the Union won. This win meant:
This early victory established Union control of the Missouri River and helped douse attempts to place Missouri in the Confederacy. It was especially important since most of the Southern sympathizers in Missouri were in the Missouri River Valley – precisely where Lyon [the Union commander] won the battle.
Here's a link to the site, E History Archive, where I got that above information. They have a great time line of events that happened during the war.