The microscopic mechanism for polarization fatigue in ferroelectric oxides has remained an open issue for several decades in the condensed matter physics community. Even though numerous models are proposed, a consensus has yet to be reached. Since polarization reversal is realized through ferroelectric domains, their behavior during electric cycling is critical to elucidating the microstructural origin for the deteriorating performance. In this study, electric field in situ transmission electron microscopy is employed for the first time to reveal the domain dynamics at the nanoscale through more than 103 cycles of bipolar fields. A novel mechanism of domainfragmentation is directly visualized in polycrystalline [(Bi1/2Na1/2)0.95Ba0.05]0.98La0.02TiO3. Fragmented domains break the long-range polar order and, together with domain wall pinning, contribute to the reduction of switchable polarization. Complimentary investigations into crystal structure and properties of this material corroborate our microscopic findings.