American Military History - Book 6: The Mexican War

Receiving by the new telegraph the news that James K. Polk had been elected to the Presidency in November 1844, President John Tyler interpreted the verdict as a mandate from the people for the annexation of Texas, since Polk had come out strongly in favor of annexation. On March 1, 1845, Congress jointly resolved to admit Texas into the Union and the Mexican Government promptly broke off diplomatic relations. President Polk continued to hope that he could settle by negotiation Mexico’s claim to Texas and acquire Upper California by purchase as well. In mid-June, nevertheless, anticipating Texas’ Fourth of July acceptance of annexation, he ordered Bvt. Brig. Gen. Zachary Taylor to move his forces from Fort Jesup on the Louisiana border to a point "on or near" the Rio Grande to repel any invasion from Mexico...