As early as 1823, America placed its eyes on other countries. In his famous Monroe Doctrine, President James Monroe stated,
"The US accepted the responsibility of being the protector of independent western nations . . . "
Though originally intended to prevent further European colonization of Latin American countries, it developed into a justification for the United States to interfere in the affairs of nations worldwide. We feel it our sworn duty to defend all nations, large or small, whether they want us to or not. Some nations, though, are threatened with being overrun by fanatical, religious factions or governments so far removed from our ideals of freedom that we feel compelled to protect said nations from falling into their hands in order to protect ourselves. In a sense, then, we are really looking out for #1 (us).
Due to worldwide Spanish aggression in the 1890's, the United States was thrust into the Spanish American War. The Spanish were eventually defeated, and the spoils of war included several islands in the Caribbean and Pacific, as well as several countries worldwide. Rather than give these people back their own governments, we elected to maintain them as "possessions," feeling they weren't capable of or didn't want to govern themselves. Our "Mother Hen" philosophy also revolved around the notion that by protecting them we felt more protected also.
Was this imperialistic policy helpful or hurtful? It ended up being both. By protecting Latin American countries for years, we helped their economies thrive, provided humanitarian services for their citizens, and insured peaceful living conditions. But, several countries have resented us for it for decades, eventually demanding their rights, and only recently having attained their freedoms as separate and independent nations.
By trying to prevent imperialism from spreading throughout the world, the United States enacted a political policy that ended up being responsible for its participating in the very thing it was trying to prevent.