Final answer:
The primary motivation for American migration to the Oregon Country was the rich soil of the Willamette River Valley, which promised fertile agricultural land. The allure of this possibility outshone other potential reasons such as the fur trade, conflict with the British, or the discovery of minerals, which were not the main drivers of settlement at the time.
Step-by-step explanation:
The significant wave of American migration to the Oregon Country during the 19th century, commonly referred to as "Oregon Fever," was primarily motivated by the promise of rich soil found in the Willamette River Valley. Pioneers were attracted by the prospect of fertile land suitable for agriculture, which played a pivotal role in fulfilling the burgeoning American ideology of westward expansion and Manifest Destiny. This migration was part of a larger context of territorial claims, colonization, and the diverse motivations of settlers, including economic opportunity and the spread of Protestant ideals.
Most Americans who migrated did not go with intentions to engage in the fur trade, which was more associated with British interests and the Hudson Bay Company's efforts. Nor were they primarily induced by conflicts with the British or the discovery of precious metals, as the famous gold rushes occurred later. Thus, the perception of Oregon as a land of fertile soil and economic promise was the major pull factor leading to the establishment of a significant American presence in the region and strengthening U.S. claims over it.