Lath martensite plays the most critical microstructural role in commercial steels used in strength requiring applications. A deeper understanding of the governing factors of its plastic behavior, especially in presence of pre-cracks, is not yet fully available. Employing in-situ scanning electron microscopy deformation experiments with microstructure, surface topography and strain mapping (i.e. using electron backscatter diffraction (EBSD), secondary electron imaging, and microscopic-digital image correlation (μ-DIC), respectively) in samples with and without focused ion beam milled cracks, we address this problem. Results of the experiments and accompanying crystal plasticity simulations unravel the underlying factors in the heterogeneous plastic accommodation in martensitic steels, and especially the key roles of local differences in crystallography, lath size and orientation, defect and boundary densities.