Manifest Destiny. beleif held in 19th century by many Americans that U.S. was destined to (meant to) span from Atlantic Ocean to Pacific Ocean, spreading its culture and democracy; belief had political, economic, and social roots:growth in U.S. economy increased demand for (and value of) farmland, ranches, and furs; the cotton gin increased the area in which cotton could be grown profitably; the discovery of gold in California attracted 80,000 people in1849