Historical herbarium collections and genetic analyses indicate that slender false brome (Brachypodium sylvaticum) was first introduced in test gardens in Oregon in the early 1900 s as part of the USDA’s plant introduction program. A small number of naturalized populations were established, but it was not until several decades later that this alien species became an aggressive invader. The Oregon invasive strains of false brome were generated as a consequence of mating among genetically divergent lineages. The resulting hybrid populations contained high levels of genetic variation that fueled the evolution of specific adaptations to the Pacific Northwest climate and ultimately generated genetically superior lineages. Although the false brome invasion has caused significant ecological and economic harm and is expected to continue spreading across western North America, understanding the circumstances that have promoted its success may provide valuable lessons for the management of native plants under pressure from global climate change.