The Fugitive Slave Act went into effect in 1850. However, it was considered a very controversial law as anyone living in the United States at that time was supposed to abide by it. The idea was that a slave could escape, the south completely admitted this, but that if the slave was found and captured, they were to be immediately returned to whomever they had escaped from. It didn’t matter if they were found in Alabama or New York. They were to be returned.
The people in the northern territories and states didn’t like this law much. It was seen as a ruthless way to keep hold on a tradition that was slowly eroding away and a way to keep change at bay.