Even after gaining new territory stretching to the Pacific Ocean, there was far more room to expand into in Northern America.  The Slave economy of The South depended greatly on its warmer climate, and the Missouri Compromise Line placed a political limit on new Northern Slave States.  There would always be more room for Free States than Slave States.  Many thought that heralded the end of Slavery in the long run.  Others forecasted the end of the United States.