The Mexican-American War was primarily triggered by the annexation of Texas by the United States in 1845. Texas had declared independence from Mexico in 1836 and had effectively operated as an independent republic for several years. However, its annexation by the United States was seen as a direct violation of Mexican sovereignty, leading to tensions between the two countries. Additionally, border disputes and conflicts over territory further escalated hostilities and ultimately led to the outbreak of war in 1846.
