Final answer:
Lincoln believed that the 'peculiar and powerful interest' that caused the war was slavery.
Step-by-step explanation:
According to Lincoln, the 'peculiar and powerful interest' that was 'somehow the cause of the war' refers to the issue of slavery. Lincoln believed that the institution of slavery was the underlying cause of the American Civil War. Slavery created deep divisions between the Northern and Southern states, with the North advocating for its abolition and the South fighting to preserve it. This clash of interests eventually culminated in the war.
Learn more about The cause of the American Civil War