California's entry into the union was denied due to the political tensions between the free and slave states in the United States in the mid-19th century. California had requested to enter the union as a free state, which would have upset the balance of power between the free and slave states. To prevent this, Congress passed the Compromise of 1850, which allowed California to enter the union as a free state, but also required the creation of the territories of New Mexico and Utah, both of which allowed slavery.