Those are two separate questions. Arguably, America has always been an empire. While I personally don't necessarily agree with it many academics (notably my history of american foreign policy professor) do. The Mexican American war as well as the wars against the Native Tribes can be seen as wars of conquest.Many believers in Manifest Destiny thought the entire western hemisphere was to be ours. Manifest Destiny's Underworld: Filibustering in Antebellum America by Robert E. May is a great book on the filibusters who after the Mex-American War sought to conquer else where for the USA. Now establishing an overseas empire came with the need for coaling stations as well as guano. The american quest for Cuba began long before the Spanish American War, as several failed filibustering expeditions tried to take the island. Southerners prior to the civil war saw it as a place to expand, and with it strengthen slavery (Slavery would not be abolished in Cuba until the 1880s). The Spanish-American war and the annexation of Hawaii began america's overseas empire. The war with Spain was driven by public outrage over the USS Maine explosion (which may have been an accident caused on board the ship) as well as yellow journalism by newspapers (Notably Hearst). After the S-A war, The Philippine war against Philippine nationalist would continue. The new empire now consisted of the Philippines, Gaum, Hawaii, PR, American Samoa, and a vassal state in Cuba. They were a part of strategy to engage China economically, which is why the US pushed for the open door policy in China. While American bases in the pacific allowed America to project power in East Asia from Pearl Harbor and Subic Bay. While our new bases (Guantanamo and San Juan Harbor) in the Caribbean allowed the US navy to control the Western Hemisphere, especially after the Panama canal was built. Now the US was brought into world war one by Uboat attacks and economic reasons. Trade with Europe was too important to be cut off. The French and British were also financially on the brink in 1917 and the US held large amounts of their debt. All of the great powers were embroiled in the great war and sitting on the sidelines meant that the US would be locked out of any post war changes in the balance of power. The Americans would tip the scales against the Central Powers as the war had become one of attrition.

I don't necessarily agree with the term empire for the rise of the USA for a variety of factors. Since the American west had such a low population density, the flooding of the west with settlers was bound to happen. The growth of America was based on unifying ideology instead of a unifying ethnicity or race. The fact is that although the natives were subdued, the U.S. Saw to incorporate and treat new states as equal members of the Union , not as subjugated territories at the whim of the other states. Was racism present? You bet native Americans didn't receive much of a say. That said the mass migration from Europe was driven by economic factors that have nothing to do with race. Much like illegal immigration today. Also the tribe never were truly sovereign in the way modern states are.. But the american republic prior to the Mexican American war had only expanded by purchasing land. The m-a war was caused by a voluntary annexation of Texas, which the Mexican government had invited Americans to settle. In fact many Tejanos fought against the Mexican government as well. (Mexico was breaking at the seams with multiple Mexican states in open revolt around the same time) . In fact America turned down an annexation treaty with the Dominican Republic in 1873. Only in the 1890s seeking to secure new markets in China and elsewhere did america have imperial ambitions. The need for coaling stations and access to Chinese and Latin American markets drove American caused her to have imperial ambition.