Nonviolent campaigns often extend beyond national boundaries, attracting the attention and involvement of international actors. However, our understanding of the effects of external support on the outcomes of nonviolent protests remains limited. Existing research mainly focuses on internal factors of success or concentrates on one or few cases. This paper aims to provide a generalizable theory of the effects of Black Knights – foreign powers supporting an incumbent government against opposition at home – on movement success. Through a large- N, quantitative analysis based on cross-sectional time-series data from 1980 to 2013, I find support for the hypothesis that foreign sponsorship negatively impacts the prospects of nonviolent victory. Additionally, this paper tests the mechanisms theorized against the case of the Belarusian uprising of 2020-2021. By formulating an extensive theory of the effects of Black Knights and adding depth to the measurement of external assistance by taking into account different types of support, my research expands the current knowledge base on the impacts of support to regimes.