Final answer:
The Oregon Country was purchased by the United States from Britain, formalizing U.S. claims up to the 49th parallel after negotiations in 1846.
Step-by-step explanation:
The United States had to purchase the Oregon Country from Britain. Following a period of joint occupation, U.S. President James K. Polk, an expansionist, negotiated with Great Britain and ultimately the two nations agreed on the 49th parallel as the boundary, thus the Oregon Territory was split between the U.S. and Britain. This agreement, formalized in June 1846, granted the U.S. claim up to the 49th parallel, establishing what is now the border between the state of Washington and Canada. The acquisition of the Oregon Country was seen as an important aspect of Manifest Destiny and was crucial for maintaining the balance between free and slaveholding states, as well as for securing a Pacific port for forthcoming trade with Asia.