Final answer:
The South became the Cotton Kingdom due to Eli Whitney's invention of the cotton gin in 1793, which made the mass production of short-staple cotton profitable, triggering a significant increase in cotton production and bolstering the Southern economy.
Step-by-step explanation:
The South became known as the Cotton Kingdom in the early nineteenth century primarily because of Eli Whitney's invention of the cotton gin in 1793. This invention revolutionized the production of cotton by rapidly separating the seeds from the short-staple cotton, which was otherwise arduous to clean by hand. Compact and robust, the cotton gin enabled planters to grow and process short-staple cotton profitably even in the inland areas where long-staple cotton, with its easily removed seeds, could not thrive.
With the advent of the cotton gin, cotton production boomed in the deep South, strikingly increasing from about 73,000 bales in 1800 to 730,000 bales in 1820. The fertile lands of the Gulf Coast states were rapidly developed for this purpose. The global demand for cotton, fueled particularly by Great Britain's textile industry, made cotton a highly profitable crop and cemented its place as the cornerstone of the Southern economy. As a result, the reliance on a single cash crop eventually led to an increased dependency on slavery, transforming the social and economic landscape of the South and contributing to the rise of the 'Slave South'.