Final answer:
The Philippines helped the United States during the Spanish-American War to assert its independence from Spain, expecting that this support would lead to self-rule after the defeat of the Spanish. However, the reality was different as the U.S. took control, and full independence for the Philippines only came in 1946.
Step-by-step explanation:
The Philippines helped the United States fight the Spanish during the Spanish-American War because it was trying to assert its independence from Spain. The Filipino leaders, especially Emilio Aguinaldo, believed that aiding the U.S., it would hasten their liberation from Spanish rule and ultimately lead to their sovereignty. Aguinaldo and other Filipino revolutionaries were disappointed when the United States took control of the Philippines instead of granting them the independence they sought after Spain's defeat.
Many Filipinos at the time saw the conflict as an opportunity to rid themselves of Spanish oppression with the expectation that they would be allowed to govern themselves afterward. The disillusionment came with President William McKinley's decision to govern the islands until the U.S. deemed the Filipinos capable of self-rule, which led to further conflict between Filipino nationalists and American soldiers.
Independence for the Philippines was not realized until July 4, 1946, after a period of Commonwealth status and the turmoil of World War II. The quest for independence and the assumption that collaborating with the U.S. against Spain during the 1898 war would facilitate this goal is a pivotal element of Filipino-American history.