Final answer:
The U.S. increased its expansionism efforts after 1898 due to nationalism, the closing of the frontier, and the desire to protect economic interests abroad.
Step-by-step explanation:
The U.S. increased its expansionism efforts after 1898 for several reasons. Firstly, there was a growing sense of nationalism and desire for global power among Americans. Secondly, the closing of the frontier in the late 19th century led to a search for new territories to expand into. Finally, the U.S. sought to protect its economic interests abroad and gain access to new markets for trade.