The biggest army in WW2 was the 14th Army, predominantly made up of men from the Indian subcontinent, also East and West African units, all were volunteers.
We paid the Chinese, you don’t pay slaves.

Voluntered by their superiors and it was Chinese Companies we paid who indentured and trapped the men in to working in a war zone.
As WW said Imperialisam is a difficult subject.....
Some folks are born made to wave the flag
Ooh, they're red, white and blue
And when the band plays "Hail to the chief"
Ooh, they point the cannon at you, Lord
It ain't me, it ain't me, I ain't no senator's son