Why does Flash think war over slavery is inevitable in the novel, Flashman & the Angel of the Lord: From the Flashman Papers, 1858-59?

Given the regional differences between North and South and the sanctimony about liberty in the U.S. Constitution, Flash thinks war over slavery is unavoidable. He says there are folks on both sides willing to compromise in order to preserve unity, but on the whole the North is strengthening its opposition to slavery and the South is digging in its heels.