American Civil War: The Battle of Westport is the last significant engagement west of the Mississippi River, ending in a Union victory.

The American Civil War was a civil war in the United States between the Union and the Confederacy, which was formed in 1861 by states that had seceded from the Union. The central conflict leading to war was a dispute over whether slavery should be permitted to expand into the western territories, leading to more slave states, or be prohibited from doing so, which many believed would place slavery on a course of ultimate extinction.

The Battle of Westport was a significant engagement of the American Civil War fought on October 23, 1864, in modern Kansas City, Missouri. Union forces under Major General Samuel R. Curtis decisively defeated an outnumbered Confederate force under Major General Sterling Price, forcing his army to retreat and marking the turning point of his Missouri Expedition. The battle ended the last major Confederate offensive west of the Mississippi River, and for the remainder of the war the United States Army maintained solid control over most of Missouri. The clash was one of the largest to be fought west of the Mississippi River, with over 30,000 men engaged.

The Mississippi River is the primary river of the largest drainage basin in the United States. It is the second-longest river in the United States, behind only the Missouri. From its traditional source of Lake Itasca in northern Minnesota, it flows generally south for 2,340 mi (3,770 km) to the Mississippi River Delta in the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains all or parts of 32 U.S. states and two Canadian provinces between the Rocky and Appalachian mountains. The river either borders or passes through the states of Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana. The main stem is entirely within the United States; the total drainage basin is 1,151,000 sq mi (2,980,000 km²), of which only about one percent is in Canada. The Mississippi ranks as the world's tenth-largest river by discharge flow, and the largest in North America.