I am pretty certain that it was Missouri.
What Was The Border Between Slave And Free States West Of The Mississippi River?
The modern interpretation of the Mason-Dixon line was the border you ask about. The Missouri Compromise and subsequent legislation apparently allowed Missouri to be a slave state.
There were actually three categories of states prior to the US Civil War: Free states, slave states and those territories that tolerated slavery. California, Oregon and Washington were the only states or territories in the West which were considered to be completely free, since slavery was permitted in New Mexico, Kansas, Utah and Nebraska.
When new states to the west of the Mississippi were admitted as states, there was no clear precedent as to whether or not to admit them as slave or free states. So when Missouri was admitted into the Union, Congress passed the Missouri Compromise. Missouri was admitted as a slave state, Maine formed as a free state, separating from Massachusetts, and future states would be free if they were north of Missouri's southern border. If they were south of the southern border, they would be slave states.
The western border is one of the great contributing factors to the American Civil War because the developing controversy in the 1850s focused on the concept of popular sovereignty--the idea that a territory/state should be able to choose on its own whether to be a "free" or a "slave" state. Initially seized upon by Northern politicians, the idea would come to head in the Kansas-Nebraska act which successively repealed the Missouri Compromise.
Passed in 1854, this act designed by famous Lincoln opponent Stephen Douglas was intended to further the idea of popular sovereignty in newly-establish western territories. This may lead to some of the confusion of your question since, strictly speaking, these decisions involved *territories* and not states. In any case, Kansas was eventually admitted as a free state and Nebraska was not admitted until after the Civil War; the furthest state to the west that would permit slavery was, therefore, Texas.
Keep in mind, however, the infamous Dred Scott Decision, through which the Supreme Court effectively ruled that a slave could still be held in bondage even in a free state or territory; there was every potential, therefore, for slavery to extend as far west as the country itself.
Passed in 1854, this act designed by famous Lincoln opponent Stephen Douglas was intended to further the idea of popular sovereignty in newly-establish western territories. This may lead to some of the confusion of your question since, strictly speaking, these decisions involved *territories* and not states. In any case, Kansas was eventually admitted as a free state and Nebraska was not admitted until after the Civil War; the furthest state to the west that would permit slavery was, therefore, Texas.
Keep in mind, however, the infamous Dred Scott Decision, through which the Supreme Court effectively ruled that a slave could still be held in bondage even in a free state or territory; there was every potential, therefore, for slavery to extend as far west as the country itself.