Final answer:
The phrase "Go West, young man" was initially coined by John Babsone Lane Soule and popularized by Horace Greeley. Greeley's endorsement of Western expansion as a safety valve reflects his significant influence on 19th-century American politics, including his presidential candidacy and views on slavery.
Step-by-step explanation:
The expression first used by John Babsone Lane Soule was "Go West, young man, go West." This phrase was later popularized by Horace Greeley, who slightly rephrased it in an editorial he authored for the New York Tribune on July 13, 1865. Greeley was a prominent journalist and also a presidential candidate in 1872. His advocacy for Western expansion was based on the belief that it would serve as a safety valve for the overcrowded cities in the Eastern United States. The theory suggested that this expansion would alleviate the strain on urban employment and housing, potentially leading to better wages and working conditions due to reduced labor supply in the cities.
Greeley's influence extended beyond promoting westward expansion; he is also known for his correspondence with President Abraham Lincoln and his involvement with political matters such as the peace movement during the Civil War and his eventual presidential candidacy against Ulysses S. Grant. Greeley's positions and actions had significant impacts on issues ranging from Reconstruction to slavery, as he often used his platform to express his views on these critical subjects of his time.