The United States provoked Mexico into the Mexican-American War by annexing the Republic of Texas in 1845, which Mexico considered to be its territory. The United States also sent troops into the disputed territory between the Nueces River and the Rio Grande, which further escalated tensions between the two countries. In addition, the United States refused to pay Mexico for damages caused by American citizens in Mexico, and the American government made demands on Mexico that were seen as unreasonable and insulting. All of these factors contributed to Mexico's decision to declare war on the United States in 1846.