After the Revolution, the United States set about becoming a new nation with George Washington becoming its first president. The leaders of the states created a constitution in 1787 and a Bill of Rights in 1791. These were based on the idea of "social contracts". In the early 1800s, the new nation faced many controversial issues, such as slavery.
During the 1800s, the United States gained much more land in the West and began to become industrialized. In 1861, several states in the South left the United States to start a new country called the Confederate States of America. This caused the American Civil War. After the war and reconstruction, many people immigrated to the United States from Europe, looking for work. Some Americans became very rich in this Gilded Age and the country developed one of the largest economies in the world.