Since the early days of Jamestown colony, Americans were constantly stretching their boundaries to encompass more territory. When the United States government was formed, the practice continued. The first half of the 19th century was spent defining the nation's borders through negotiation and war, and the second half was spent populating the fruits of the labor. As the 20th century dawned, many believed that the expansion should continue.
Many different groups pushed for American expansion overseas. Industrialists sought new markets for their products and sources for cheaper resources. Nationalists claimed that colonies were a hallmark of national prestige. The European powers had already claimed much of the globe; America would have to compete or perish. Missionaries continually preached to spread their messages of faith. Social Darwinists such as Josiah Strong believed that American civilization was superior to others and that it was an American's duty to diffuse its benefits. Alfred Thayer Mahan wrote an influential thesis declaring that throughout history, those that controlled the seas controlled the world. Acquiring naval bases at strategic points around the world was imperative.
Before 1890, American lands consisted of little more than the contiguous states and Alaska. By the end of World War I, America could boast a global empire. American Samoa and Hawaii were added in the 1890s by force. The Spanish-American War brought Guam, Puerto Rico, and the Philippines under the American flag. The Roosevelt Corollary to the Monroe Doctrine declared the entire western hemisphere an American sphere of influence. Through initial negotiation and eventual intimidation, the United States secured the rights to build and operate an isthmathian canal in Panama. The German naval threat in World War I prompted the purchase of the Virgin Islands from Denmark in 1917.
The country that had once fought to throw off imperial shackles was now itself an empire. With the economic and strategic benefits came the expected difficulties. Filipinos fought a bloody struggle for independence. America became entangled with distant conflicts to defend the new claims. Regardless of the nobility or self-interest of the intent, the United States was now poised to claim its role as a world power in the 20th century.