Adams–Onís Treaty
The Republic-claimed borders were based upon the Treaties of Velasco between the newly created Texas Republic and General Santa Anna, who had been captured in battle. The eastern boundary had been defined by the Adams–Onís Treaty of 1819 between the United States and Spain, which recognized the Sabine River as the eastern boundary of Spanish Texas and western boundary of the Missouri Territory. Under the 1819 Adams–Onís Treaty, before Mexico's 1821 independence, the United States had renounced its claim to Spanish land to the east of the Rocky Mountains and to the north of the Rio Grande, which it claimed to have acquired as part of the Louisiana Purchase of 1803.