Among the several factors that led to American expansionism was the idea of Manifest Destiny--that America was destined to overspread the continent. Although this originally applied only to North America, the doctrine was quite easily extrapolated to include the rest of the world. Combine this with the belief that America was destined to become a world power, which it could not do without some overseas interests; and the idea that as a Christian nation, it had an obligation to spread the gospel abroad, and one gets a good grasp of the reasons behind American expansionism.
The impact on society as a whole was mixed. Many were caught up in great patriotic fervor, believing that America had fulfilled its destiny by becoming a world power. This idea was enthusiastically endorsed by the British Poet, Rudyard Kipling, in his poem, The White Man's Burden:
Take up the White Man's burden--
Send forth the best ye breed--
Go, bind your sons to exile
To serve your captives' need;
To wait, in heavy harness,
On fluttered folk and wild--
Your new-caught sullen peoples,
Half devil and half child.
Still others, including Mark Twain, Andrew Carnegie, and Samuel Gompers, opposed expansion, particularly the thought of colonizing the Philippine Islands which the U.S. gained under the Treaty of Paris of 1898. They and a number of others formed the American Anti-Imperialist League, urging that all overseas expansion be ceased. Twain expressed the sentiment best in To The Person Sitting in Darkness a response to Kipling:
The more we examine the mistake, the more clearly we perceive that it is going to be bad for the Business. The Person Sitting in Darkness is almost sure to say: "There is something curious about this – curious and unaccountable. There must be two Americas: one that sets the captive free, and one that takes a once-captive's new freedom away from him, and picks a quarrel with him with nothing to found it on; then kills him to get his land.
For many, the justification for expansionism was its own reward; yet for others, American had become drunk with power and rather than a leader, had become an international bully.