As the British discovered trying the quell the American rebellion, it’s difficult to fight the natives on their own territory, even when many of them are on your side. The US faced the same situation in Vietnam, its difficulty magnified because Americans were of a different racial stock than those they subjugated.
Also unlike the British, the Americans were perceived—even by their ostensible allies—as another in a long line of imperial conquerors that stretched back to Chinese domination during the first millennium. Belying US rhetoric and propaganda, industrial warfare and atrocities destroyed South Vietnam and killed or alienated many of the South Vietnamese whose freedom we were supposedly defending.
To defeat a local population when much of it wages guerrilla war or covertly supports those who do, the invading power has to kill most and terrorize the rest.