After Mexico declared its independence from Spain, tensions arose between the United States and Mexico. Before the Mexican people declared their independence, the treaty between the leaders of the United States and Spain in 1819, the Adams-Onis Treaty, had established that Mexico's border lay along the Sabine River. In 1821, the governments of the United States and Mexico signed a treaty stating that both governments will undertake to maintain peace and harmony among the Native Americans living along the border between the two countries. However, due to the Sabine River's unique geography, the United States and Mexico had different interpretations of where the border actually was. This led U.S. leaders to claim the city of Nacogdoches as their own, while the Mexican government believed it was in their territory. When the President of the United States sent General Gaines to occupy Nacogdoches, anti-American sentiment was unleashed that would fuel the Mexican-American War. There are many underlying causes of the Mexican-American War, but one of the primary causes was the U.S. government's redefinition of the terms of a treaty to its own advantage. Throughout history, US leaders have redefined the terms of treaties with other countries and peoples, such as the Treaty of Alliance with France and the Treaty of Fort Laramie, to benefit the expansion of US territory. The primary causes of the occupation of Nacogdoches were the treaties made by U.S. leaders with both the leaders of Spain and Mexico. The US government was trying to expand US territory, which led to tensions with Mexico. Texas represented a particularly difficult obstacle between the United States and Mexico because there were many U.S. settlers in Texas. US settlers outnumbered Mexicans in Texas and many residents of Fort Laramie to keep the peace, as long as it was convenient for US citizens. The 1821 treaty between Mexico and the United States, the Treaty of Alliance with France, and the Treaty of Fort Laramie revealed that the United States government defied or interpreted the terms of the treaty to its own advantage. The ideals that the United States represents – such as truth, justice and freedom – are called into question when we look at the means by which the United States became the greatest power in the world. Works Cited Alan Axelrod, American Treaty and Alliances, (Washington, DC: A Division of Congressional Quarterly Inc., 2000) Robert, L., and Pauline Jones. “Nacogdoches Occupation.” East Texas Historical Journal, 50 (2012), n.2: 16-40Justin H. Smith, War with Mexico (Gloucester: The Macmillan Company, 1963)Alan Axelrod, Political History of America's Wars, (Washington, DC: CQ, 2007)