The Spanish-American war gave the United States for the first time an overseas Empire. Unable to pay the indemnity for the war, Spain withdrew from Cuba and gave the Philippines to the United States to pay for the war.
U.S was pursuing an 'Open Door' policy which aimed at obtaining a wide range of economic opportunities in Asia. The ideas of an 'informal empire' were being pushed by Alfred Thayer Mahan who wrote a very influential book in which suggested the U.S should obtain an island chain of colonies for economic growth.
The Spanish-American War Proved to other european powers that The United States were most certainly an emerging imperial power and not just someone to mess with.