Board Thread:Theories and Speculation/@comment-27924662-20160917205629/@comment-734582-20160918171218

The sharingan can see chakra, it stands to reason it can see Danzo charge his eye, and notice the disturbance in Obito. Obito suddenly attempting suicide out of nowhere would be noticeable by Sasuke in an instant.

Izanagi wouldn't have mattered: Obito made it clear he would intervene the moment Danzo came close to winning. It doesn't matter whether Sasuke would be drained by a lengthier fight: Obito would strike Danzo down