Complex sentence: The 1807 Act ended the legality of all international slave trade with the U.S. However, it was not always well enforced, and slavery itself continued in the United States until the end of the Civil War and the adoption of the 13th Amendment to the Constitution.
Split into simpler sentences: The 1807 Act ended the legality of all international slave trade with the U.S. However, it was not always well enforced. The institution of slavery continued in the United States until the end of the Civil War and the adoption of the 13th Amendment to the Constitution.