The US certainly DID become an Empire (imperialist nation) when it acquired Spain's holdings after the Spanish War (1898). They took the Philipine Islands, Guam and the Mariannas Islands, Puerto Rico, Cuba, and so much more! In a separate action, Sanford B. Dole had deposed the Hawaiian queen and had applied for, and gotten, US annexation.
The American Empire was huge, and there would soon be a canal through a new country called "Panamerica" or Panama for short that would serve to connect the two lobes of the American empire.
As for why the US wanted an Empire, it was for mixed reasons. Part of the reason was that they had been goaded into the Spanish War by publishers Hearst and Pulitzer with their yellow journalism and jingoist slogans. These urged America to not be pushed around by Spain (it wasn't trying) and to free the poor citizens of Cuba.
Just like today, the war was easy to win, but the US had no idea how to deal with the aftermath. Just like today, there were many people in the conquered countries that didn't WANT the US there at all! There are many parallels between McKinley and Bush, and between the Spanish War and the Second Gulf War.
