Liu Bei attacked Yi Ling after the death of Guan Yu (who was defending Jingzhou, territory that rightfully belonged to Wu but was instead under the Shu banner).
At Yi Ling Liu Bei suffered a crippling defeat that may have been the reason why Shu couldn't compete against Wei in later campaigns. Many talented military commanders were killed and many lives were lost. This campaign also, indirectly, resulted in the deaths of Zhang Fei and Liu Bei himself, many would say.
The Yi Ling campaign also damaged Wu. It is clear that, in order to survive against the great power of Wei, that Shu and Wu should have been cooperating instead of fighting against each other.
My question is this: Why did Liu Bei attack Wu? Was it revenge for the death of Guan Yu? Was he just looking for an excuse to attack Wu? Let us know what you think.
I personally think it was a combination of the death of his brothers and the anger of having lost Jingzhou. I certainly do not think it was a good decision, even if he had been successful. Consider even a success? Was Shu in a condition to govern so much territory against Wei? I feel that Shu and Wu needed each other to stand a chance.