True.
Because, if slavery was NOT expanded, the abolitionists would have control of Congress. When that state finally occurred, the representatives of these “Free” states imposed high tariffs and excises on overseas trade - which mostly impacted the South and favored Northern industries.
When faced with the reverse - a independent South with a 10% tariff, the North saw grass growing the deserted streets of New York and Boston - and no alternative but war.
They had a far more compelling problem than that. If slavery were not expanded to new markets, two things would have happened because of the rapidly growing slave population. First, the price of slaves would have collapsed wiping out the biggest source of capital in the South. Second, they would eventually be hopelessly out numbered by slaves with the very real possibility of Southern States ending up like Haiti.
The entire John Brown scenario was the worst fear for the old slave masters.
Which was why the South opened fire on US forces at Ft. Sumter.