<P> The primary catalyst for secession was slavery , especially Southern political leaders ' resistance to attempts by Northern antislavery political forces to block the expansion of slavery into the western territories . Another explanation for secession , and the subsequent formation of the Confederacy , was white Southern nationalism . The primary reason for the North to reject secession was to preserve the Union , a cause based on American nationalism . Most of the debate is about the first question , as to why some southern states decided to secede . </P>
what was the immediate catalyst to the civil war
slavery