Which event occurred first?

the Louisiana Purchase

the Missouri Compromise

the 1855 Kansas election

the Kansas-Nebraska Act

What came first was the Louisiana Purchase. Napoleon sold Louisiana to the United States when he failed reestablish an extension of the Empire after getting it from Spain. This gave the Americans more territory and many Americans wanted to move west. After that was Missouri Compromise where the balance between slave state and Free state was maintained. With the creation of the Kansas Nebraska Act, the Missouri Compromise was nulled as an effect. This would later affect the Kansas elections and later lead to Civil War.