During the early 20th century, U.S. Navy and Marine Corps leaders studied ways to defend America, win fleet battles, and seize beachheads but ignored a key emerging method of conducting military operations: sustained overseas expeditionary warfare.
American foreign and military affairs underwent revolutions at the turn of the 20th century, but the country's Army, Navy, and Marine leaders failed to fully comprehend the nature of the changes. During its first 125 years of independence, the United States had largely focused its attention on North America and adjacent areas. American expeditionary operations in the Mexican War and Civil War took place close to U.S. ports and bases and were therefore far easier to execute and sustain than operations outside North American waters. In 1898, however, the United States declared war against Spain and U.S. troops crossed the Pacific Ocean to fight in the Philippines.