The study of hydrogen embrittlement of materials has not as of yet led to methods that can be used to design safe and reliable hydrogen containment components in which hydrogen-induced degradation is associated with subcritical crack growth. For example, current design guidelines for pipelines only tacitly address subcritical cracking by applying arbitrary, conservative safety factors on the applied stress. In an effort to understand the mechanics of sustained-load cracking in a gaseous hydrogen environment, we investigate crack propagation in the Fe-Ni-Co superalloy IN903 for which experiments by Moody et al.  suggest that hydrogen promotes failure by intergranular cracking at pressures greater than 100MPa. In the present work, we model the hydrogen effect on grain boundary cohesion through the thermodynamic theory of Mishin et al.  which addresses transient separation accounting for the rate of hydrogen diffusion into the separating interface. We calibrate the parameters of the theory through first-principles calculations and information from the experiments and we simulate crack propagation by cohesive finite element methodology. The results reveal a number of issues related to the complexity of the failure mechanism and the robustness of the cohesive element methodology.