Well, perhaps for the Americans it was, although they cheated because imperialism or colonialism was viewed negatively from an international standpoint, but they managed to annex Hawaii (undemocraticaly) and to keep Puerto Rico as a colony disguising it for a Common Wealth ( in the fifties). They helped Cuba out in their independence, but managed it through its puppet government, as well as some other Latin American countries. They also controlled the Philippines, despite the fact that they achieved their independence. The American Virgin Islands, Guam, Mariana islands are still colonies, or were until just recently.
It may have been legitimate at the turn of the 19th century, but by the mid 1900s, there was much international pressure to solve the colonial situation because it was viewed as immoral: people have a right to self-government and or sovereignty. Yet, the Americans managed to cheat this.
