How did the admission of new territories and states increase the tension between free and slave states?

In the history of the United States of America, a slave state was a U.S. state in which the practice of slavery was legal, and a free state was one in which slavery was prohibited or being legally phased out. Historically, in the 16th and 17th century slavery was legal throughout British Empire. It existed in all the British colonies of North America. In the Thirteen Colonies, the distinction between slave and free states began during the American Revolution (1775–1783). Slavery became a divisive issue and was the primary cause of the American Civil War. The Thirteenth Amendment to the United States Constitution, ratified in 1865, abolished slavery throughout the United States, and the distinction between free and slave states ended.