Containment

Yesterday in class we discussed the issues that the United States faced with slavery and what was attempted to be done with it. As mentioned slavery was a pressing problem and few were sure what should be done with it. Most people agreed that they wanted it to cease to spread. Instead of outright ending it, they wanted to contain it to the South. This became impractical due to the Manifest Destiny Spirit that was sweeping the nation. The question I pose is, why; if containment didn't work with the South, did they think it would be successful to contain communism to the Soviet Union? Also, why did it work?