At the time, the U.S. felt that their imperialistic policies were justified. Whether people today agree or disagree is another question. But in the late 1800’s and early 1900’s, the U.S. felt it was necessary for several reasons. There were economic considerations. Many felt that the United States economy could only grow if it gained new foreign markets and raw materials from foreign acquisitions. Building up trade also meant having a strong navy to control the world’s sea lanes, which also meant having bases and fueling stations throughout the Pacific. Another justification for imperialistic policies was the idea of that the U.S. held a special place in the world and that it was an obligation of the U.S. to spread its ideals to the rest of the world. These were some of the justifications for U.S. imperialism in the late 1800’s and early 1900’s.
Of course, this is simply a matter of opinion. And it must be said that it is a complex question as well since the US undertook imperialism in a wide variety of areas, from Latin America all the way out to the Philippines.
I would tend to say that yes, the United States was justified. I have two general reasons for saying this.
First, if the US had not been imperialistic, the places it took would not have remained free (in my opinion). For example, if the US had not annexed Hawaii, it seems likely that the British would have. Or if the US had allowed the Philippines to become independent, it seems likely that some other country would have taken those islands. So it is not as if only the US stood between various areas and their freedom.
Second, although it is not "correct" to say so, I think that many of these areas were not really ready for self government. (I am of Filipino descent so I think I should be exempted from charges of racism when I say this.) If the US had left these countries alone, would the lives of their people have been better? It seems unlikely to me. So I do not believe that the US hurt these countries by acting imperialistically.
While the Washington Post detected in an article on the Spanish-American-Cuban-Filipino War "the taste of empire . . . in the mouth of the people", several intellectuals of the period spoke up against American imperialism. These included Mark Twain, William Jennings Bryan, Jane Addams and philosopher William James. Several of those who thought the United States was not justified in pursuing an imperialist course cited the Declaration of Independence and the Constitution. Imperialism implied the conquest of people against their will and thus it violated the value of self-determination that had given birth to the United States. James went as far as saying that the United States was betraying its cultural and historical heritage.
Another argument against imperialism was that the acquisition of new markets could take place without necessarily imposing American rule on foreign countries. Those speaking out against imperialist policies were also worried that because the President as Commander in Chief could dispatch troops overseas without seeking Congressional approval, the constitutional checks-and-balances system would be easily subverted.
I think that any nation's relationship to imperialism and colonialism is directly dependent on how one views the reality of the time period. From the point of view who were colonized, I think a strong case could be made that the United States was not justified. Given the amount of public dissent, while not overwhelming but still present, that was in America at the time, a case to that effect could still be made. I think that the government felt that it was in their interest to pursue imperialistic policies in at the turn of the century for a variety of national security and economic security based reasons. Whether or not one accepts these might be another issue. Much of the foreign policy that drove expansion was influenced heavily by economic interests and business voices of the time period. Some have argued that this might not provide justification for the investment of foreign policy and military endeavor, while others argue that such moves were of vital importance to the sustaining of the new nation. I think that, as previously noted, how one feels on this depends largely on the criteria used for assessing whether or not the business element is an appropriate rationale for expansion.
In my opinion, those areas of the world that the U.S. dominated, would have been dominated by another imperialistic country if the U.S. had not dominated them. But, if the U.S. had of stayed out of the imperial business, its plutocrats would have been sufficiently wealthy anyway; what they wanted was to be sinfully wealthy.
However that may be, the U.S. did have extensive trade relations that did not involve imperialism (if you disregard the imperialistic, domestic-industry-protecting-tariff that was imposed upon American consumers), and the sea routes over which this trade passed, needed to be protected. Some of the islands in the Pacific, that the U.S. acquired, were acquired for harbours and coaling stations for U.S. merchant ships and the U.S. war ships that protected them.