What affects did the fugative slave act have on the civil war?

It helped to cause it.

The Act put ordinary citizens under an obligation to report anyone who looked as though they might be a runaway slave, on pain of a heavy fine.

The public responded by setting up the Underground Railroad - a sysem of safe-houses by whch fugitive slaves could be smuggled into Canada.

Harriet Beecher Stowe wrote 'Uncle Tom's Cabin' as her own angry response to the Act. It brought many new converts to the cause of Abolitionism, and generally raised the temperature of the debate.