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A modern martensitic stainless steel (Ferrium® PH48S™) resists hydrogen environment assisted cracking (HEAC) in aqueous NaCl at ultra-high yield strengths (1,400 MPa to 1,600 MPa). HEAC is transgranular, because of increased steel purity and La addition, compared to severe intergranular HEAC in Custom® 465-H900 without rare earth elements. Minimum threshold for HEAC (KTH) is low (8 MPa√m to 17 MPa√m) for each steel under substantial cathodic polarization. Transgranular HEAC occurs along martensite packet and {110}α′-block interfaces in PH48S, likely a result of H decohesion enabled by localized plasticity. Martensite transformation produces a large area of coincident site lattice interfaces in the refined microstructure of PH48S. However, a susceptible network of random packet/block interfaces is connected in 3D to limit interface engineering. Nanoscale strengthening precipitates in PH48S reduce effective H diffusivity to the mid-10−10 cm2/s range, because of reversible H trapping with a binding energy of 12 kJ/mol. This diffusivity reduces the Stage II growth rate by 1 to 3 orders of magnitude compared to C465 and carbide strengthened ultra-high strength steels. PH48S and C465 are nearly immune to HEAC when cathodically polarized by 50 mV to 500 mV, attributed to a minimum in occluded-crack tip overpotential for H production. The breadth of this protective-potential window increases with decreasing steel strength. Increased Cr does not degrade HEAC resistance, suggesting that crack passivity dominates cation acidification to reduce H production and/or uptake. A quantitative decohesion model effectively predicts the potential dependence of da/dtII using crack tip H solubility reverse calculated from a KTH model.