During the last deglaciation, the Barents-Kara Ice Sheet (BKIS), a marine-based sector of the Eurasian Ice Sheet, was subject to a drastic retreat over only a few centuries. While the timing of the BKIS deglaciation is well documented, the mechanisms driving the ice-sheet retreat remain debated. Using the GRISLI2.0 ice sheet model, we investigate the behavior of BKIS during this period and identify the marine ice sheet instability (MISI) as the primary driver of the BKIS collapse. Contrary to current interpretations found in the literature, which suggest that a MISI is primarily initiated by ocean-induced basal melting, our results suggest that surface processes, particularly atmospheric warming, can directly trigger such a dynamic instability. Our results highlight the combined roles of atmospheric and oceanic forcings, with atmospheric warming triggering the initial retreat at the onset of the deglaciation and oceanic processes subsequently controlling its dynamics. We therefore encourage future studies on marine ice sheets instability, to give a better consideration to variations in atmospheric conditions on their impact on ice sheet destabilization.