The desire for the admission of Missouri as a slave state caused so much conflict because it threatened to upset the balance of power between slave and free states in the United States Senate. At the time, there were 11 free states and 11 slave states, and the admission of Missouri as a slave state would have tipped the balance in favor of the slave states. This was seen as a threat to the rights and interests of the free states, and it led to a fierce debate in Congress over whether Missouri should be admitted as a slave state or a free state. The debate was eventually resolved by the Missouri Compromise of 1820, which admitted Missouri as a slave state but also admitted Maine as a free state, thus preserving the balance of power in the Senate.