Large wildfires with uncharacteristically high severity are occurring more frequently in western U.S. forests. The increasing size and severity of wildfires has been attributed to both an increase in weather conducive to fire spread and changes to forest structure and fuel loads due to management practices that included fire suppression over the previous century. Fuel reduction treatments aim to produce a more fire-resistant forest structure by reducing densities of small, shade-tolerant trees and surface fuel loads. Despite the wealth of information demonstrating reduced wildfire severity in areas with completed fuel reduction and restoration treatments, there is still uncertainty in the ability of these treatments to affect wildfire severity outside their footprint (i.e., landscape-scale effect). This is particularly true under more extreme burning conditions that often occur during days when large wildfires undergo rapid growth. We conducted a series of studies to understand the influence of fuels treatments and previous wildfires, along with fire weather, vegetation, and topography, on fire spread and severity in the 2013 Rim Fire. We first performed a validation of classified fire severity using field plots collected pre-and post-Rim Fire in which we found that fire severity classes represented distinct states of vegetation change, with high severity being indicative of stand replacing fire. Assessing burn patterns across the Rim Fire, severity tended to be lowest in areas that had previously been treated with prescribed fire, and highest in areas that had previously burned in a high severity wildfire. Previous low to moderate severity wildfire was associated with lower fire severity in the Rim Fire, and was included as a fuel treatment in analyses of the landscape-level effects of fuel reduction. For sample landscapes within the Rim Fire footprint, there was a negative relationship between the incidence of high severity fire and the proportion previously treated or burned at low to moderate severity. High and moderate severity fire was reduced as the Rim Fire moved from untreated into treated or previously burned areas. Daily fire weather had a strong impact on fire severity in the Rim Fire, with large areas burned at high severity occurring when the burning index and energy release component were relatively high. This “overriding” influence of weather may have been influenced by fuel conditions across the larger landscape. Fire severity across the fire footprint showed a high degree of spatial autocorrelation, suggesting a “momentum” effect in which fire severity was not immediately responsive to changes in fuels or topography. Pre-fire vegetation generally did not improve our ability to predict Rim Fire severity, but higher fire severity was associated with areas with a high density of small trees, a greater abundance of fir, and a greater amount of standing dead biomass. Together, our findings imply that fuel reduction did reduce the incidence of high severity fire in the Rim Fire, but some fuel treatments had little effect compared to the overriding conditions of fire weather and the contagious nature of fire spread. Areas burned at high severity are vulnerable to long-term forest loss through type conversion, while areas burned at moderate severity can lead to mixed effects in a subsequent fire, depending on the resulting vegetation and fuel dynamics. In contrast, areas burned at low severity in either wildfire or prescribed fire were likely to reburn at low severity, increasing the resilience of the landscape to wildfire.