Those involved at the time of U.S. Expansionism had no problems justifying it; however in the wisdom of hindsight, it is hardly justifiable.
The belief at the time was that U.S. civilization and culture was superior to others, Social Darwinism epitomized. The then belief was that the U.S. had a duty as a Christian nation to "civilize" other nations. This was primarily an excuse, although most people of the day bought into it wholesale. The United States had overspread the North American continent pursuant to a policy of "Manifest Destiny," that is God willed that America do just that. Carrying that culture to the rest of the world seemed the next logical step.
The true reasons were a bit more mundane. Admiral Alfred Thayer Mahan in his book, The Influence of Sea Power on History had recommended that the U.S. develop a modern navy and also obtain a coaling station in the Pacific as well as build an Isthmian Canal. The Canal was built through Panama after the U.S. rather shamelessly interfered in a revolt of the Panamanians from Colombia. The coaling station was, of course Hawaii, where American sugar magnates had overthrown the rightful queen with the help of U.S. Marines, established the Republic of Hawaii with a constitution which specified that the country would seek annexation by the United States. At the time, the U.S. minister to Hawaii had sent a telegram to Washington stating.
The Hawaiian pear is now fully ripe and this is the golden hour for the United States to pluck it.
There were economic reasons also:Exports of farm products had been the basis of American economic growth. The original colonies only survived on the basis of exporting tobacco, indigo, lumber, etc. to Europe. Throughout the 19th century, cotton, corn, wheat, etc. had been exported. Now, technology had led to improved methods of transportation and communication, commerce moved much more quickly. There was a growing belief that American manufacturers had grown to the point that they too could compete, perhaps even outsell foreign manufacturers in the World Market.
U.S. involvement in the Spanish American War, which John Hay called a "splendid little war" and gained the U.S. the Philippines, Guam, and Puerto Rico, was based on the assumption that Spanish agents had sunk the U.S.S. Maine, which was untrue.
So, can it be justified? Only if self interest is considered a proper motive.
When looking at the justified reasons for American imperialism, through 21st century lenses, it easy to break down the argument for imperialism. However, what were the objectives of policy makers leading America at that time? Competing economically, on an international scale, was at the forefront of the argument that would make American imperialism necessary, in order to meet those economic goals and compete on a global scale.
Great Britain, France, Germany, Japan, and Russia had been implementing imperialist policies throughout the 19th century and influential thinkers of the time gave similar reasons to justify those policies. Thinkers such as John Hobson & Vladimir Lenin spoke of the problematic issues caused by overproduction, which had been caused by the rapid innovations of the industrial worlds, and one of the remedies to that problem was to colonize outside of a nation’s borders. Through colonization, the dominant imperialist countries could create vast increases in their markets to help alleviate overproduction. So if American policy makers implemented the popular 19th century idea of Realpolitik, which stresses the practical and realistic need and concerns facing nations, those leaders could easily justify imperialism over the ideals of human rights and self-determination. However, some Americans would not have been satisfied with a purely economic justification for imperialism, so pro-imperialists had to create other avenues to push their agenda.
As mentioned in the prior post, Social Darwinism, spreading Christianity, and Mahan’s The Influence of Sea Power upon History all played a supporting role for the pro-imperialist cause. One might also examine the thesis presented in Frederick Jackson Turner’s “The Significance of the Frontier in American”, where Turner argues that territorial expansion promotes social, economic, and political stability, so with the frontier now “closed”, Americans had to look beyond North American borders to stabilize the country. Domestically, there was economic and social instability at the turn of the century, and policy makers had to find ways to stabilize the country. An imperialist policy would help bring the economy out of immediate crisis caused by the Panic of 1893, help create conditions that would allow for future investments and help reduce class conflicts by reducing unemployment, passing on economic benefits to more Americans, and utilizing the passion of patriotism to mute the voice of class conflict.
Again, imperialism is not acceptable in the modern world despite the numerous amounts of arguments which could be posed, but we should avoid applying modern views when judging actions of the past. However, if the main goal of American leaders and policy makers was to make the United States a world power and empower the economy, than one could have a strong argument that imperialism was justifiable as a way to meet that goal.