The U.S. Environmental Protection Agency took an unusually strong step Jan. 31 and blocked a proposed Alaska mine heralded by backers as the most significant undeveloped copper and gold resource in the world. The EPA based its veto on concerns over the mine’s potential environmental damage to Alaska lands and waters that support the world’s largest sockeye salmon fishery.

The move, cheered by Alaska Native tribes and environmentalists and condemned by some state officials and mining interests, deals a heavy blow to the proposed Pebble Mine. The intended site is in a remote area of south...