Short answers, pulled from the story.
Manifest destiny was the 19th-century American belief that settlers were destined to expand westward across North America, with that expansion seen as both obvious ("manifest") and certain ("destiny"). The phrase was first used in print by John O'Sullivan in his 1845 essay "Annexation" in the Democratic Review, though some historians credit journalist Jane Cazneau as the actual author of that unsigned editorial.
Most historians credit John O'Sullivan, a newspaper editor and advocate for Jacksonian democracy, with coining the term in 1845. However, some historians argue the unsigned editorial where it first appeared was written by journalist Jane Cazneau. O'Sullivan's second use of the phrase, in the New York Morning News on the 27th of December 1845, was the one that made the term widely known.
Manifest destiny provided ideological cover for the displacement of Native American peoples from their lands. Federal policy, shaped largely by Henry Knox and Thomas Jefferson, pursued land acquisition through treaties, while the Indian Removal Act of 1830 formalized removal policy. The Homestead Act of 1862 led to the settlement of over 270 million acres, directly displacing Indigenous communities; the U.S. Army, under generals including Sherman, Sheridan, and Custer, waged wars against tribes who remained on ceded lands.
No. Abraham Lincoln opposed the Mexican-American War and rejected manifest destiny as both unjust and a threat to the moral bonds of liberty and union. He argued that the imperialism of manifest destiny, along with anti-immigrant nativism, disordered genuine patriotism; his 'Eulogy to Henry Clay' on the 6th of June 1852, is considered the most sustained expression of his alternative, reflective patriotism.
Manifest destiny was inseparable from the slavery debate because every new territory raised the question of whether slavery would be permitted there. The annexation of Texas was controversial because it added a slave state; the Mexican-American War deepened the division; and former president John Quincy Adams reversed his earlier support for expansionism by 1843 specifically because expansion meant the spread of slavery into Texas.
In the 1890s, manifest destiny was cited to justify annexation of Hawaii, the Philippines, Puerto Rico, Guam, and American Samoa. President McKinley used the phrase explicitly when advocating for Hawaii in 1898. The Spanish-American War of 1898 then transferred the Philippines, Puerto Rico, and Guam from Spain to the United States under the Treaty of Paris, with the United States paying Spain $20 million for the Philippines.