Comment by graemep

Locals are not a uniform group.

1. They might want allies against their enemies 2. They might be subjects of an existing empire and prefer a new empire 3. They might be an oppressed ethnic or religious group 4. They might be a class that wants to take power from the ruling class 5. Empires typically allow freedom of trade and freedom of movement within them. People will want this. 6. They might be employed by the new power.

Lots of examples. Indian bankers financing the East India Company and even encouraging them to overthrow some rulers. The cast majority of both East India Company troop and, later British troops in India were Indians. People subjugated by the Aztecs allied with the conquistadores, lower caste Indians might well have got a better deal from the British, lots of people migrated within the British Empire for better opportunities, lots of people traded within the Empire too.

People from one colony might be used in another - IIRC the Portuguese used African mercenaries in South Asia.