The two causes of the Mexican-American War are:
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Annexation of Texas - The annexation of Texas by the United States in 1845 was a significant trigger for the war, as it upset Mexico and was viewed as a loss of territory.
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Dispute over the boundary between Texas and Mexico - There was a disagreement over where the boundary between Texas and Mexico should lie, particularly whether it was the Rio Grande or the Nueces River, which escalated tensions and contributed to the outbreak of war.
The Treaty of Guadalupe Hidalgo and the California Gold Rush were consequences of the war rather than direct causes.