The reigns of Abu Bakr, Umar, and Uthman as the first three khalifas (caliphs/successors) of Muhammad instead of Ali are one of the main point of contention in the Sunni/Shia split.
I wonder if Ali gave allegiance to (or otherwise accepted the reigns of) these three caliphs? Did he do this freely or under duress? There wasn't any major conflict started by Ali during these reigns, and "the establishment" eventually elected Ali as the fourth caliph after Uthman, so it seems that he got along fine?