Following the Spanish-American War in 1898, the US paid Spain $20 million for the Philippines, initiating American colonial rule. Filipinos, who initially saw Americans as allies for independence from Spain, were met with an American desire to "uplift and civilize and Christianize them." The US established a military government in Manila, setting up local governments, schools, and courts. Concurrently, the Philippine revolutionary government formed during the Spanish-American War, adopted a constitution on January 21, 1899, establishing the First Philippine Republic. However, on January 4, 1899, the US declared immediate military rule over the entire archipelago, undermining the new Philippine government. President Emilio Aguinaldo reaffirmed Philippine independence, leading to open warfare on February 4, 1899, with an American assault on San Juan.

Aguinaldo's government declared war on the US on June 2, 1899. Facing a modern military, Filipinos shifted from traditional warfare to guerrilla tactics, aiming for constant losses rather than direct confrontation. This prompted the Americans to adopt counter-insurgency methods, including civilian internment camps, which led to widespread disease and more Filipino deaths than direct combat. Martial law was declared in December 1900 by General Arthur MacArthur, while a US commission designed and enacted a new government. The capture of President Aguinaldo in March 1901 significantly reduced hostilities, though isolated resistance continued until 1913. The war was costly for both sides, with the US incurring significant expenses and the Filipinos losing at least 200,000 civilians to disease and famine due to American scorched-earth policies and brutal interrogation tactics. Filipino fighters also used brutal tactics against American captives.

After the war, the Philippines became an Insular Government, administered by the US with the goal of eventual independence. An assembly was elected in 1907, forming the lower house of a bicameral legislature, which annually called for independence. The Philippine Autonomy Act (Jones Law) of 1916 replaced the US-appointed commission with an elected senate and affirmed the American policy of independence, though without a timeline. Independence efforts were paused during WWI but resumed afterward. While President Woodrow Wilson believed the Philippines were ready for independence in 1921, his successors disagreed. It wasn't until January 1933 that the US government allowed for a timeline, established by the Philippine Independence Act of 1934. This act provided guidelines for a constitution and a commonwealth, with independence scheduled for July 4, 1946, following the tenth anniversary of the Commonwealth's establishment on November 15, 1935.

The new Philippine government's ambitious plans were hampered by the Great Depression and the rising threat of Japan. Concerns proved valid when Japan attacked the Philippines on December 7, 1941, simultaneously with Pearl Harbor. The Philippine Army was integrated into the US Army Forces Far East under General Douglas MacArthur. Manila was declared an open city and captured by the Japanese on January 2, 1942. Philippine President Manuel L Quezon and Vice President Sergio Osmeña established a government-in-exile in Washington, D.C. The Japanese set up a puppet government, tempting some Filipinos with the promise of independence, but most sought genuine independence. Poor preparation by the Americans led to their planes being destroyed on the ground, leaving American and Filipino forces vulnerable. They retreated to Corregidor Island and eventually surrendered, leading to the infamous Bataan Death March. The Japanese, who did not value surrender, treated prisoners brutally, resulting in an estimated 600 American and 15,000 Filipino deaths during the march. The Japanese occupation was brutal, involving forced labor and the use of women and children as "comfort women," fuelling a fierce, widespread underground resistance movement.

In October 1944, General MacArthur returned to the Philippines. Philippine guerrillas had significantly weakened Japanese control and provided vital intelligence to the Americans. MacArthur, influenced by his and his father's involvement with the Philippines, felt a strong personal commitment to its liberation. The pivotal Battle of Leyte Gulf (October 23-26, 1944), the largest naval battle in history, involved complex Japanese naval maneuvers and American responses, ultimately securing a decisive Allied victory. The Americans retook Manila in a brutal street-by-street battle from February 3 to March 3, 1945, with tens of thousands of civilians killed. Fighting continued until Japan's surrender on September 2, 1945. A few months later, on November 15, 1945, the Philippine Commonwealth celebrated its tenth anniversary. Official independence was granted with ceremony on July 4, 1946.