The Oregon Treaty of June 15, 1846, resolved the Oregon boundary dispute by dividing the Oregon Country/Columbia District between the United States and Britain "along the forty-ninth parallel of north latitude to the middle of the channel which separates the continent from Vancouver Island, and thence southerly through the middle of the said channel, and of the Strait of Juan de Fuca, to the Pacific Ocean."
However, there are actually two straits that could be called the middle of the channel: Haro Strait, along the west side of the San Juan Islands; and Rosario Strait, along the east side.
In 1846, there was still some uncertainty about the geography of the region. The most commonly available maps were those of George Vancouver, published in 1798, and of Charles Wilkes, published in 1845. In both cases the maps are unclear in the vicinity of the southeastern coast of Vancouver Island and the Gulf Islands. As a result, Haro Strait is not fully clear either.
In 1856, the US and Britain set up a Boundary Commission to resolve a number of issues regarding the international boundary, including the water boundary from the Strait of Georgia to the Strait of Juan de Fuca. The British appointed James Charles Prevost First Commissioner, George Henry Richards Second Commissioner, and William A. G. Young Secretary. The US appointed Archibald Campbell First Commissioner, John Parke Second Commissioner, and William J. Warren Secretary. On June 27, 1857, the American and British commissioners met for the first time on board the British ship HMS Satellite, anchored in Esquimalt Harbour. The two sides met several more times in 1857 in Esquimalt Harbour and Nanaimo Harbour, and corresponded by letter between meetings. The water boundary was discussed from October to December. From the start, Prevost maintained that Rosario Strait was required by the treaty's wording and was intended by the treaty framers, while Campbell had the same opinion for Haro Strait.
Prevost held that the channel specified in the treaty must have three key qualities:
- it must separate the continent from Vancouver Island,
- it must carry the boundary in a southerly direction, and
- it must be navigable.
Only Rosario fulfilled these requirements, he wrote. Campbell countered that the expression "southerly", in the treaty, was to be understood in a general sense, that Rosario Strait did not separate the continent from Vancouver Island, but the San Juan Islands from Lummi Island, Cypress Island, Fidalgo Island, and others, and that navigability was not germane to the issue, but even if it was, Haro Strait was the wider and more direct passage. Finally he challenged Prevost to produce any evidence showing that the treaty framers had intended Rosario Strait. Prevost responded to the challenge by referring to American maps showing the boundary running through Rosario Strait, included one by John C. Frémont, produced for and published by the US government, and another by John B. Preston, Surveyor-General of Oregon in 1852. To the other points, Prevost repeated his statements about Rosario Strait's navigability—the channels between Lummi, Cypress, and Fidalgo islands not being navigable—and that a line through Rosario would be southerly, while one through Haro would have to be drawn westerly. The two continued to discuss the issue into December 1857, until it was clear what each side's argument was and that neither would be convinced of the other. Prevost made a final offer at the sixth meeting, December 3. He suggested a compromise line through San Juan Channel, which would give the US all the main islands except San Juan Island. This offer was rejected and the commission adjourned, agreeing to report back to their respective governments. Thus ambiguity over the water boundary remained.
Because of this ambiguity, both the United States and Britain claimed sovereignty over the San Juan Islands. During this period of disputed sovereignty, Britain's Hudson's Bay Company established operations on San Juan and turned the island into a sheep ranch. Meanwhile, by mid-1859, twenty-five to twenty-nine American settlers had arrived.
San Juan Island held significance not for its size, but as a military strategic point. While the British held Fort Victoria on Vancouver Island to the west, overlooking the Strait of Juan de Fuca, the entry point to Haro Strait, leading to the Strait of Georgia, the nation that held the San Juan Islands would be able to dominate all the straits connecting the Strait of Juan de Fuca with the Strait of Georgia.
General George B. McClellan, George Pickett’s West Point Classmate and lifelong friend, claimed that General William S. Harney and Pickett conspired with a cabal, to start a war with Britain, creating a common enemy, to head off a north-south confrontation. However, General Granville O. Haller debunked McClellan's theory. He said they had wanted to start a war, but with hope of distracting the north so that the south could gain independence.
The theories are given credence when it is noted that Major General Silas Casey, then a Lieutenant Colonel and deputy commander of the 9th Infantry Division, was reduced to a support role for Captain George Pickett who was given independent jurisdiction over a vast area by Harney, then a Brevet Major, and was also passed over by Harney in favor of Pickett when given this choice command.
On the other hand it can be said that Lt Col Casey had not been reduced, for he was given command over the USS Massachusetts and Major Haller to protect and supervise the water of Puget Sound. He was given discretion to deviate from his orders, based on his military experience. (His textbook on infantry tactics was used at West Point during the Civil War).