Final answer:
After the joint occupation of Oregon began in 1818, few settlers migrated to the area until the early 1840s, when "Oregon Fever" led to a significant westward migration. This influx of American settlers strengthened the U.S. claim to Oregon, fueling the idea of Manifest Destiny. Eventually, an agreement was reached in 1846 to divide Oregon at the 49th parallel.
Step-by-step explanation:
In the years after the 1818 "Joint occupation" of Oregon began, few Americans and Europeans settled in the area. However, starting in the early 1840s, there was a significant westward migration known as the "Oregon Fever."
Thousands of American settlers traveled to Oregon Territory over the Oregon Trail, which helped strengthen the U.S. claim to Oregon. By the mid-1840s, around 5,000 Americans had populated the Territory.
Furthermore, the idea of Manifest Destiny, which promoted the belief in American expansion, was popularized through the settlement of Oregon. Eventually, negotiations between the United States and Great Britain resulted in an agreement in 1846 to divide Oregon at the 49th parallel, with the southern portion belonging to the United States.