Answer:
The United States Declaration of Independence spoke of universal rights for all, but this did not prevent Thomas Jefferson, the chief author of the Declaration, from owning slaves; though he believed that slavery would gradually disappear with natural evolution. However, slavery continued to exist in the southern states of the United States until 1865, the end of the Civil War.
At the height of slavery in the late 18th century, the abolitionist movement against it began in Britain. This movement was initially based on Quakers who chose to regard slavery as non-Christian, and secular thinkers influenced by enlightenment. The anti-slavery movement expanded and spread to France and elsewhere. British economic influence, along with their own anti-slavery movements, led most northern US states, as well as many other countries, to ban slavery.
Abolitionism began in the northern United States shortly after independence. By 1804, the abolition of slavery had already been decided in all the northern states of the so-called Mason-Dixon line, although there had already been relatively few slaves. Conversely, in the southern states of the line, slave labor was heavily used for decades, especially for the needs of the expanded cotton industry in the 19th century.
In 1807, a law was passed prohibiting the introduction of new slaves into the United States. The law came into force early next year. However, the birth rate of slaves was clearly higher than mortality. As a result, the number of slaves continued to grow sharply: by 1830 there were two million slaves in the United States, but by 1860 there were four million.
When Abraham Lincoln, known as an anti-slavery president, was elected President of the United States in the 1860 elections and had promised that no new state would become a slave state, the southern states, the Confederacy, declared independence and the Civil War began.
