What makes synthetic aperture radar (SAR) automatic target recognition (ATR) hard? This question is explored by reviewing target, environment, and sensor variability and how they affect SAR images and target recognition in SAR images. Each of these categories of operating conditions (OCs) is reviewed first with a wide open 'real world' scope and then comparing that to the extensive MSTAR SAR data collections. The target OC review considers increasingly fine target categories, from class to type to versions, and then configuration, articulation, damage, and moving-part variants. The environment OC considers topological properties, that might affect 6-DOF pose and terrain obscuration, volumetric (vegetation, snow, ...) and surface scattering properties, and occlusion, layover, and adjacency issues. The sensor OC review is limited to outlining the important properties of SAR imaging systems, such as their variation in frequency, PRF, BW, polarization, depression, squint, SNR, etc. SAR and optical images are used to illustrate OC dimensions. This review is limited to open literature sources and is from an ATR rather than a domain expert perspective. However, this space of SAR ATR OCs will eventually need to be understood before we will truly know the SAR and ATR problem.