Upon the annexation of Texas (in 1845) Mexico at once severed her diplomatic relations with the United States. This result had been foreshadowed by the utterances of Mexican officials dating from the revival of the question in 1843. The relations, however, of the two countries had been difficult to adjust from the time when Mexico became independent in 1821. The most serious friction between them arose concerning four subjects: claims of the United States citizens on the government of Mexico; assistance given the Texans by the people of the United States; violation of Mexican territory by United States troops; and the annexation of Texas. * * * * * The immediate occasion, however, of the breach of diplomatic relations in 1845 was the annexation of Texas. When rumors of the renewal of the annexation movement came to the city of Mexico in the summer of 1843, President Santa Anna gave notice to the United States government, in a letter dated August 23d, from Secretary of State Bocanegra to Minister Waddy Thompson, that “the Mexican government will consider equivalent to a declaration of war against the Mexican Republic the passage of an act for the incorporation of Texas with the territory of the United States; the certainty of the fact being sufficient for the immediate proclamation of war, leaving to the civilized world to determine with regard to the justice of the cause of the Mexican Nation, in a struggle which it has been so far from provoking.”[101] Thompson replied immediately with a sharply resentful letter, questioning the sources of information of the Mexican authorities as to the prospect of annexation, but refusing any explanation whatever. Another letter from Bocanegra to Thompson asserted that the advices of the Mexican government on the subject were official and reliable, and sought to justify the attitude of Mexico as follows: “but as it may happen that ambition and delusion may prevail over public propriety, that personal views may triumph over sane and just ideas, and that the vigorous reasoning of Mr. John Quincy Adams and his co-laborers may be ineffectual, how can it be considered strange and out of the way that Mexico, under such a supposition, should announce that she will regard the annexation of Texas as an act of declaration of war?”[102] Secretary of State Upshur approved the course of Thompson, and instructed him that, in case he were again addressed in such offensive language, he should demand either a withdrawal of the letter or a suitable apology.