Encouraged by the notion of manifest destiny, U. S. territory expanded all the way to the Pacific coast. While the United States was large in terms of area, its population by 1790 was only 4 million. However, it grew rapidly, reaching 7. 2 million in 1810, 32 million in 1860, 76 million in 1900, 132 million in 1940, and 321 million in 2015. Economic growth in terms of overall GDP was even greater. However compared to European powers, the nation's military strength was relatively limited in peacetime before 1940. The expansion was driven by a quest for inexpensive land for yeoman farmers and slave owners. The expansion of slavery was increasingly controversial and fueled political and constitutional battles, which were resolved by compromises. Slavery was abolished in all states north of the Mason–Dixon line by 1804, but the South continued to profit from the institution, mostly from production of cotton. Republican Abraham Lincoln was elected president in 1860 on a platform of halting the expansion of slavery.