actually,we didn't "Formally" attack on 21st Nov.we covertly helidropped some 3000 SFF commandos to Demagiri,a border village who started hit and run.by 21st,SFF commandos already entered into Chittagong Hill Tract by crossing the river and engaged PA.
Actually,3rd December's raid was a masterpiece of Indian Policy makers.They actually prepared ground for this attack,using
1971 Indian Airlines hijacking - Wikipedia, the free encyclopedia and following overflight ban.The attack formally started the war,with negligible damage that pre-emptive attack has caused..But India prepared itself for moths about this attack.They prepared themselves for months and waited for this attack.and after formal declaration of war,simply attacked both fronts..