The Oregon Treaty was an agreement between Britain and the United States. It came into force on 15 June 1846. It formalized the border between the United States and British North America west of the Rocky Mountains. It extended the border along the 49th parallel to the Pacific Ocean and down “the middle” of the channel that separates Vancouver Island from the mainland. The treaty resolved an important dispute between the two nations. But the lack of a precisely defined “middle” led to a dispute over the San Juan Islands, which resulted in an 1859 diplomatic conflict known as the Pig War.