Complex sentence: After the American Revolutionary War, Pennsylvania passed the Gradual Abolition Act (1780), the first of any in the new United States, which declared as free children born to slave mothers after that date.
Split into simpler sentences: After the American Revolutionary War, Pennsylvania passed the Gradual Abolition Act (1780), the first such law in the new United States. Pennsylvania's law established as free those children born to slave mothers after that date.