In no particular order: No, green cards cannot be revoked without process. That process happens in immigration courts, which are part of the Justice Department, but decisions appealable to federal courts. Yes, ICE can detain people in "non-punitive" detention while they are in removal proceedings
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Once you concede an inch of the argument to the fascists, you already lost.
1) Have no real charge
2) No one to know where he has been taken?