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As wildland fire activity continues to surge across the western US, it is increasingly important that we understand and quantify the environmental drivers of fire and how they vary across ecosystems. At daily to annual timescales, weather, fuels, and topography are known to influence characteristics such as area burned and fire severity. An understudied facet, however, concerns how these factors inhibit fire spread and thereby contribute to the formation of fire boundaries. We evaluated how weather, fuels, and topography impeded fire spread in four large study areas in the western US, three in the Northern Rockies and one in the Southwest. Weather and fuels were the most important factors in the Northern Rockies, whereas fuels and topography were dominant in the Southwest. Within the categories of weather, fuels, and topography, we also evaluated which specific variables were most influential in impeding fire …