Dementor question

[michaelkgidlow]

No: HPFGUIDX 78569

Something has been bugging me about the Dementors. Were they allied 
with Voldemort during his first rise to power (before Harry was 
born)? Because if they were, then why were they put in charge of 
Azakaban? Maybe they had always been guards at Azakaban and were 
secretly allied with Voldemort. But if that's the case, then why at 
the end of the second book did they so openly leave their posts at 
Azakaban? And what does it mean when they are "sent for" or "sent 
back" to Azakaban? I'm confused. Would someone shed some light on 
this?

Michael Gidlow.