Mofras, wrote in 1840 "...it is evident that California will belong to whatever nation chooses to send there a man-of-war and two hundred men." In 1841, the Mexican military commander in Northern California, General Mariano Vallejo, wrote "there is no doubt that France is intriguing to become mistress of California." U.S. expansion In 1805, the first official party of Americans to arrive on the west coast of North America, the fabled expedition of Lewis and Clark, came down the Columbia River to the river's mouth on the border between today's Oregon and Washington. In 1819, the United States acquired the Spanish claims to the Pacific Northwest (as negotiated in the Nootka Convention) in the Adams-Onís Treaty. The United States argued that it acquired the Spanish rights to exclusive ownership of the Pacific Northwest as far north as Alaska, even though Spain had in fact relinquished any claim to exclusive rights as a result of the Nootka Conventions. This position led to a dispute with Britain known as the Oregon boundary dispute, remembered for the slogan "54-40 or fight!" The two countries agreed to defer resolution of the dispute, and to allow settlement by both British and American immigrants in what became known in the United States as the Oregon Country (today's Oregon, Washington, and Idaho; much of today's British Columbia; and parts of Montana and Wyoming). In 1841, the United States Exploring Expedition visited the west coast of North America by ship, and sent an overland party down the Siskiyou Trail, from the site of today's Portland, Oregon, to San Francisco. Americans continued arriving on the west coast of North America in significant numbers in the mid-1830s. They first came overland along the Oregon Trail, settling primarily in the rich Willamette Valley south of today's Portland. By 1841, the first overland party of American settlers reached California along what became the California Trail, and by the mid-1840s significant numbers of Americans were arriving in California. In addition, the long-standing dispute between the United States and Great Britain over the Oregon Country was resolved in 1846, with the signing of the Oregon Treaty; the Oregon Treaty divided the disputed territory along what later became the current international boundary between Canada and the United States. Rapid growth (1846–1945) In this era, much of the west coast of North America transformed from an area still largely populated by indigenous peoples to widespread population