The United States gained The Oregon Territory through an 1846 treaty with Great Britain, hence the correct option is A.1.
The 1846 Treaty
The 1846 treaty was signed between Great Britain and US. It was the manifest destiny and the interest of President James Polk that this treaty came into being.
The treaty established the 49th parallel as the border between the two countries. Since this treaty gave Oregon territory to US, it is also known as Oregon Treaty.
Therefore, the correct option is A.1.
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