Explain how and why slavery expanded over time.

Slavery started in America from the beginning of its settlement by Europeans. It went through various phases until its end at the conclusion of the Civil War.
In 1619, a Dutch ship brought the first slaves to America. The slave trade grew because slaves were seen as a cheaper alternative to the use of indentured servants. Millions of Africans were imported from Africa in the seventeenth and eighteenth centuries, and they were employed on rice, indigo, and tobacco farms in the South.
After America achieved independence from England in 1783, the US Constitution (1789) recognized and legitimized the existence of slavery and counted them as 3/5 of a person for purposes of representation and taxation. Nevertheless, by 1804, states in the North had outlawed slavery; the African slave trade was abolished 1808.
Slavery might have died out in the South had it not been for the invention of the cotton gin in 1793. In 1793, Eli Whitney invented a device to facilitate the removal of seeds from cotton. This was a boon to slavery because it made cotton production—with slave labor—much more profitable.
The number of slaves in the South increased greatly in the fifty years before the Civil War (1861–1865). Southern leaders thought their economy—even their culture—depended on slavery. They were determined to keep it and spread it wherever they could. The South's loss in the Civil War finally put an end to the odious institution of slavery in the US.

Slavery, especially in the United States, expanded due to many factors. The first factor was the large supply of arable farmland in the country, especially in the South. As demand grew for textile products worldwide, Southern planters saw an opportunity to take the cheap land and turn it into a platform for cash crops. In order to work thousands of acres without mechanized equipment, they used slaves. Before 1808, they could buy the slaves from Africa; after this year, however, the United States stopped importing slaves. The slave owners could only rely on the progeny of their slaves. By 1820, a good field hand could be as expensive as a new luxury car today if compared in today's dollars.
The cotton gin also allowed for the expansion of slavery. It allowed one slave to do the work of many and made the industry even more profitable. This allowed for more acres to be utilized in monoculture and the slave industry to grow even further. Slavery also put pressure on the United States to expand as well, as planters sought cheap land in the American Southeast and Texas.