After the spanish american war, the united states was
considered an imperialist nation. what did this mean? *

It meant a paradoxal situation because the proper foundation of the USA was an anti-imperialist and anti-colonial act. Foundation of the USA was an emancipation against British imperialism. But already in 1840s USA started practicing imperialist policy toward Mexico (Texas) and Central America. It was a paradox. Spanish American war was sort of a culmination of this kind of policy.

Explanation: One of the explanations is a Social Darwinism which was very fashionable at that time and its perspective was applied not only to societies and races but also to history. According to this perspective Latin nations were decadent and condemned to extinction. Such a view seemed to be confirmed by Franco-Prussian war (example). The same perspective followed Americans (Theodor Roosevelt).

this isn't a question and if you're going to list things be kind and use commas.

the adoption by russian princes of the mongols' diplomatic rituals