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 The poison 

When the japanese diplomat replaced huo's tea with the poisoned one, did Anno Tanaka realise at the end of the movie? To me, it is strongly implied as a yes. There is evidence that supports it, and is strongly shown when the japanese diplomat said ""Who say?, Who saw it?"" after Tanaka gave the victory to Huo when they were walking out of the arena. Anno Tanaka replied ""I saw myself"" and then went on telling him that he was a disgrace to the japanese people. Also right after Huo collapsed from his final blow, he said ""wait"" and held him, observing his face and it looks like he realised or must have seen. This also shows that he must have realised that he was poisoned. 

I am pretty sure that Anno Tanaka did know at the end."