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.This continued until gains of the Civil Rights Movement in the 1960s and passage of federal legislation to enforce constitutional rights were made.The United States became the world's leading industrial power at the turn of the 20th century due to an outburst of entrepreneurship in the Northeast and Midwest and the arrival of millions of immigrant workers and farmers from Europe.Economic growth in terms of overall GDP was even greater.
In the Reconstruction Era (1863–1877), legal and voting rights were extended to freed slaves.
After the Cold War, the United States began focusing on modern conflicts in the Middle East and nuclear programs in North Korea.
The beginning of the 21st century saw the September 11 attacks by Al-Qaeda in 2001, which was later followed by the U. In 2008, the United States had its worst economic crisis since the Great Depression, which was followed by slower-than-usual rates of economic growth during the 2010s.
Its involvement culminated in using newly invented nuclear weapons on two Japanese cities to defeat Imperial Japan in the Pacific theater.
The United States and the Soviet Union emerged as rival superpowers in the aftermath of World War II.