This time period really should be differentiated. The early part of the period was dominated by self-interest while the end of the period had aspects of idealism about it. Overall, I would argue that self-interest predominated, but it would be much clearer if we separated this period into the WWI Era on the one hand and the time before it on the other.
The early part of this time period is dominated by the Spanish-American War and the Roosevelt Corollary to the Monroe Doctrine. These things showed the US doing things like occupying ore even taking other countries (Nicaragua, the Philippines) to promote its own interests.
The WWI era is dominated by Woodrow Wilson and the Fourteen Points. Wilson's goal was to create a world order that would be conducive to peace. This is a much more idealistic motive than was present in the early 1900s.
Overall, I would argue that the self-interest predominated, but that idealism came more to the fore at the end of the period.
During this time period our foreign policy was imperialism, spreading to other nations. Both sides can be argued
The White Man’s Burden - Rudyard Kipling. The idea that it is the duty of the white race to help all others who aren't white because they are savages.
Religion - Many missionaries wanted to expand into other nations in order to spread Christianity
Help Western Hemisphere - Roosevelt Corollary to the Monroe Doctrine If any nation in the western hemisphere appeared politically and fiscally unstable and vulnerable to European control, the US had the right and obligation to intervene.
Reconcentration Camps - Spanish American War 1898. In 1896, Spain instituted reconcentration camps in Cuba. They forced Cubans into these camps to keep them from helping the rebels. 1/3 of people in these camps died from disease and starvation. After we won the war we gained Cuba, Puerto Rico and the Phillipines
Navy - a powerful navy was needed to protect our country and we needed ports to refuel around the world. We gained the Panama Canal by interferring with Columbia and Panama. We first tried to buy the rights to build it and were denied. Afterwards we help Panama gain independence and then had a faster way to travel.
Markets - we needed new places to sell goods too and to get natural resources from. In Hawaii, we overthrew the monarchy to keep the markets.
Power - we wanted to compete with other countries who had lots of colonies
Pride- The DeLome Letter, was written by Enrique Dupuy de Lôme, the Spanish Minister (and Cuba). In a personal letter, he stated President McKinley was weak. We soon went to war with Spain after that