<p>Interprovincial border counties disproportionately bear the burden of transboundary air pollution, yet often face weak incentives to enforce stringent environmental regulation. This study examines whether <i>China’s Three-Year Action Plan for Winning the Blue Sky Defense Battle</i> improved green development in these jurisdictions. Using a balanced panel of 1790 counties from 2012 to 2021, we treat the 2018 policy rollout as a quasi-exogenous shock and estimate a multi-period difference-in-differences model with county and year fixed effects. The results show that the policy significantly increases green development in interprovincial border counties. The estimates are robust to alternative specifications, placebo tests, propensity-score reweighting, and a double machine learning procedure that flexibly adjusts for high-dimensional confounders. Mechanism analyses indicate that the policy operates through accelerated industrial upgrading, strengthened green innovation, improved ecological quality, cleaner production substitution, and enhanced cross-provincial joint prevention and control. Heterogeneity tests reveal larger environmental governance gains in poorer counties and counties with more rugged terrain, whereas economic gains are stronger in richer and flatter counties. A triple-differences extension further suggests that coordination between the Blue Sky Defense Battle and other environmental regulations remains incomplete, generating offsetting effects in certain contexts. These findings underscore the importance of coherent policy packages and stronger cross-regional governance to facilitate green transformation in border regions.</p> Graphical Abstract <p></p>