At the end of summer 2021, 90% of the western US was in drought and more than half of the region was in extreme or exceptional drought, causing water scarcity, agricultural degradation, devastating wildfires, and associated socio-economic and ecological losses. While previous studies have attributed western/southwestern US droughts to oceanic and/or internal atmospheric variability, land-surface conditions can also influence drought onset and intensification through soil moisture memory and/or land-atmosphere interactions. The contributions of each factor to a specific drought are difficult to quantify and have not been as widely studied. Using CESM2 simulations with prescribed SST and sea ice, this study focuses on estimating the contribution of land surface conditions (vegetation and soil moisture), oceanic forcing, and internal atmospheric variability to the 2020-21 western US drought. We conduct multiple ensembles of sensitivity experiments for the period May 1^st 2020-November 30^th 2021 that differ in atmospheric initial conditions, soil moisture initializations, and vegetation variation. Our major findings are: 1) the drought initiation during the 2020 summer can be attributed to internal atmospheric variability, while the progression of the drought into the subsequent winter and spring was tied to La-Niña conditions in the tropical Pacific; 2) drought-induced decreases in LAI reduced soil water consumption during the simulated period, even though the signal is not statistically significant; 3) the initial soil moisture state significantly contributes to the initiation and development of agricultural drought, primarily due to the long soil moisture memory in this region; and 4) imposed dry initial conditions for soil moisture during spring 2020 cause more persistent agricultural drought evolution during 2020-21 than for wet initializations. This last finding has implications for predictability studies and how predictability might vary during prolonged wet or dry mean states caused by anthropogenic forcing or multidecadal variability in the Pacific and Atlantic Oceans.