This dissertation is composed of three papers and related unpublished work providing a foundation that helps understand near threshold fatigue crack growth in both vacuum and environment conditions in the atomic scale. All studies are made possible by utilization of a recent implementation of the multiscale coupled atomistic and discrete dislocation (LF-CADD) methodology. First, near threshold fatigue crack growth of a ductile material in vacuum is investigated via atomistic modeling of dislocation motions and slip interactions. The simulation results indicate that sustained fatigue crack growth in vacuum requires emitted dislocations to change slip planes prior to their reabsorption into the crack on the opposite side of the loading cycle. This finding assesses the validity of long-hypothesized material separation mechanisms using state-of-the-art computational resources, conforms to reports of crack growth below experimentally assisted fatigue crack growth thresholds, and opens the door for improved prognosis and the design of novel fatigue resistance alloys. Second, the role of material dissolution is studied for intrinsically ductile and semi-brittle materials isolated from other environmentally assisted fatigue crack growth mechanisms. The multiscale simulations and subsequent analysis suggest that dissolution can be dual natured depending on the material. For a ductile material, dissolution at crack tip acts to blunt the crack with increasing crack tip opening displacement (CTOD). On the other hand, dissolution will work as a driving force for a slow growing in a brittle and semi-brittle material where the crack remains atomically sharp. The latter will eventually trigger brittle, unstable crack propagation as the applied load exceeds a critical threshold. Third, a crack with surface film is studied under mode I fatigue loading. Using our concurrent multiscale LF-CADD model, we conduct a series of simulations with varying loading conditions and find that the crack grows via film rupture mechanism, which requires film thickness and brittleness. Surface film swelling is needed to produce experimentally observed R-effect on $\Delta K$ threshold. In order to achieve sustained fatigue crack growth, a critical K ($K_{max}$) is necessary to overcome the compressive stress ahead of the crack tip at the bottom of the loading cycle due to film swelling.