In March 1845, the U.S. Congress passed a joint resolution offering to annex the Lone Star republic, prompting the Herrera regime to sever diplomatic relations with the United States. While the government in Mexico City publicly declared that a union between Texas and the United States would constitute an act of war, by the fall of 1845 the Mexican government was seeking a face-saving alternative. Privately, Herrera sought to open a dialogue with Washington, in the hopes of negotiating a settlement that would include compensation for the loss of its former province.
When Herrera’s moderate policy became public, it infuriated Mexican centralists and puro federalists alike, who condemned the regime’s course as one of appeasement. At the same time, Herrera received little help from the Polk administration in Washington. While the administration promptly dispatched a diplomat, John Slidell, to Mexico City to initiate negotiations, it had already sent an army under Zachary Taylor to the south Texas frontier, further inflaming Mexican public opinion. Hoping the furor might die down, the Herrera regime declined to accept Slidell’s diplomatic credentials when he arrived in the nation’s capital in December.
By this time, however, conservatives led by General Mariano Paredes y Arillaga were already conspiring against the regime. Paredes seized power on January 2 1846, vowing to reassert Mexican sovereignty over Texas as far as the Sabine River. Like its predecessor, the new government in Mexico City also refused to accept Slidell’s diplomatic credentials. In March Slidell returned to the United States, all hope of a constructive dialogue between the two countries having come to an end. In Polk’s mind, Mexico’s rejection of Slidell provided "ample cause of war," and favored sending Congress a declaration to that effect. In fact, a clash between the two countries had already occurred. Shortly after meeting with Slidell, the president learned that on April 25 the Mexican army had crossed the Rio Grande and attacked a patrol of dragoons. General Taylor's note to the War Department read: "Hostilities may now be considered as commenced."
