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Using three independent ice‐flow models and several satellite‐based datasets, we assess the importance of correctly capturing ice‐shelf breakup, shelf thinning, and reduction in basal traction from ungrounding in reproducing observed speed‐up and thinning of Thwaites Glacier between 1995 and 2015. We run several transient numerical simulations applying these three perturbations individually. Our results show that ocean‐induced ice‐shelf thinning generates most of the observed grounding line retreat, inland speed‐up, and mass loss, in agreement with previous work. We improve the agreement with observed inland speed‐up and thinning by prescribing changes in ice‐shelf geometry and a reduction in basal traction over areas that became ungrounded since 1995, suggesting that shelf breakups and thinning‐induced reduction in basal traction play a critical role on Thwaites's dynamics, as pointed out by previous studies. These findings suggest that modeling Thwaites's future requires reliable ocean‐induced melt estimates in models that respond accurately to downstream perturbations. Plain Language Summary: Recent observations have shown that Thwaites Glacier, West Antarctica, has been accelerating and thinning over the past decades and its floating part is quickly breaking up. While these observations suggest that warmer ocean currents are the main factor responsible for these changes, it remains unclear which of the following processes are most important to the glacier's dynamics: (a) breakup of its floating section, (b) ice‐shelf thinning, or (c) changes in the grounded‐ice area. By employing three ice‐sheet models and several satellite‐based datasets, we find that thinning induced by ocean melting and the resulting reduction of grounded‐ice area explain most of the observed flow acceleration and mass loss of Thwaites, in agreement with other studies. We also find that the breakup of the floating section plays an important role on Thwaites's dynamics. These findings suggest that improved forecasts of Thwaites's future require reliable ocean‐induced melt estimates and improved model response to changes in ice‐shelf thickness and geometry. Key Points: The reduction in basal traction due to grounding line retreat plays a critical role on Thwaites's dynamics in agreement with previous studiesOcean‐induced melt leads to a sustained acceleration and grounding line retreat consistent with observations and earlier workImproved forecasts of Thwaites demand reliable melt estimates coupled to models that accurately reproduce the response to downstream changes [ABSTRACT FROM AUTHOR]