Over half of the acid falling on Canadian lakes came from U.S. coal-fired power plants, especially in Ohio and Illinois. The 1991 treaty between President George H.W. Bush and Prime Minister Brian Mulroney offers key lessons: Canadians’ emotional connection to their heritage, especially in Ontario and Québec, played a pivotal role; Canada acted first to reduce its contributions, making it harder for the U.S. to deflect blame; and Canadian environmental groups lobbied in the U.S., partnering with affected states to address cross-border acid rain.