Final answer:
President Lincoln responded to the Southern states seceding by proclaiming it as an illegal and unconstitutional act, stating that it weakens the country and makes it vulnerable. He took measures to suppress the rebellion and protect federal property.
Step-by-step explanation:
During the American Civil War, after the Southern states seceded from the Union, President Abraham Lincoln immediately responded by asserting that secession would weaken the country and make it a target for foreign powers. He called it an illegal and unconstitutional action and emphasized that the Union was a perpetual entity. Lincoln showcased his commitment to preserving the Union by taking action to suppress the rebellion and mobilizing troops to protect federal property.
Learn more about Abraham Lincoln's response to the Southern states seceding