CALGARY, Alberta—A day before Michigan’s deadline to close down a key crude oil pipeline, Canada on Tuesday issued its strongest remarks so far about the move, warning that it could undermine relations with the United States, its closest ally and trading partner. Canadian company Enbridge Inc is preparing for a legal battle with Michigan and courting protests from environmental groups, betting it can ignore the state’s Wednesday deadline to shut down Line 5, which runs under the Straits of Mackinac. The Canadian government, intervening in the case to back Enbridge, said in a U.S. federal court filing that Michigan had no right to act unilaterally since a 1977 Canada-U.S. pipeline treaty guarantees the free flow of oil between the two nations. “This case raises concerns regarding the efficacy of the historic framework upon which the U.S.-Canada relationship has been successfully managed for generations,” Ottawa said. Michigan’s move “threatens to undermine …