How did fugitive slave law lead to the civil war?

Not surprisingly, it angered the Abolitionists, including Harriet Beecher Stowe, who wrote 'Uncle Tom's Cabin' as her response to it.

More significantly, it offended many Northerners who did not have strong feelings about slavery, but resented being treated as unpaid slave catchers, who were required to report anyone who looked like a runaway, on pain of a heavy fine.

It encouraged the use of the Underground Railroad - the safe-house system that smuggled runaways into Canada.

And it served to drive North and South further apart than ever.