Final answer:
Lincoln imposed martial law and suspended habeas corpus in response to the attack on Baltimore, successfully preventing some border states from seceding and maintaining Union support among them.
Step-by-step explanation:
In response to the attack on Baltimore, President Abraham Lincoln imposed martial law and increased federal control to stabilize the situation. This included arresting southern-sympathizing members of the Maryland state legislature and suspending habeas corpus, to prevent them from aiding the Confederacy. As for the border states, these actions, along with military presence and political maneuvers, effectively stopped some of them from seceding. Maryland, despite criticisms of Lincoln's actions, did not secede, and approximately 66 percent of its white men fought for the Union. Other border states like Delaware, Kentucky, and Missouri also remained in the Union.