It's not entirely correct to say the Allies weren't aware, as there was intelligence and aerial reconnaissance that indicated the presence of SS heavy armour. However, it seems the operation's commanders were in denial about the risks, as they waved away the reports and the dangers posed by it. The only officer that tried to make them see the risk was packed of on medical leave.
Of course, this was just one of several major blunders in the planning of the operation.
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