Final answer:
President Abraham Lincoln was most concerned about losing Maryland to the Confederates, as it would put the Union capital at risk.
Step-by-step explanation:
During the American Civil War, President Abraham Lincoln was most concerned about the potential secession of the border state of Maryland. This concern was due to Maryland's proximity to the Union capital, Washington, D.C., which would have been surrounded by Confederate territory had Maryland joined the Confederacy. Consequently, both the Lincoln and Davis administrations sought to attract the loyalty of the so-called Border States, which included Maryland, Kentucky, Missouri, and Delaware, due to their significant natural resources, industrial capacity, and strategic importance.