Ottawa last week formally invoked the dispute-resolution article of a 1977 treaty governing transit pipelines between the two nations. The treaty states that, except in an emergency, natural disaster or pressing safety concern, “no public authority in the territory of either” the U.S. or Canada may take measures “which are intended to, or which would have the effect of, impeding, diverting, redirecting or interfering with in any way the transmission of hydrocarbon in transit.”