The President Officially Signed The Ordinance Of Annexation Of Texas
The original controversy about the legality of the annexation of Texas stems from the fact that Congress approved the annexation of Texas as a territory with a simple majority vote approval instead of annexing the land by Treaty, as was done with Native American lands. After the United States and The Republic of Texas were unable to reach a Treaty agreement, Congress passed a Joint Resolution for Annexing Texas to the United States. The Republic of Texas' Annexation Convention then submitted the Ordinance of Annexation to popular vote in October 1845 and the public approved the measure. This Ordinance of Annexation was submitted and approved by the House and Senate of the United States and signed by the President on December 29, 1845. While this was an awkward, if not unusual, treaty process it was fully accepted by all parties involved, and more importantly all parties performed on those agreements making them legally binding (see Contract Law). In addition, the United States Supreme Court decided in the case of DeLima v. Bidwell that annexation by a joint resolution of Congress is legal.
There was an ongoing border dispute between the Republic of Texas and Mexico prior to annexation. Texas claimed the Rio Grande as its border, while Mexico maintained that it was the Nueces River and did not recognize Texan independence. President James K. Polk ordered General Zachary Taylor to garrison the southern border of Texas, as defined by the former Republic. Taylor moved into Texas, ignoring Mexican demands that he withdraw, and marched as far south as the Rio Grande, where he began to build a fort near the river's mouth on the Gulf of Mexico. The Mexican government regarded this action as a violation of its sovereignty.
The Republic of Texas never controlled what is now New Mexico. The failed Texas Santa Fe Expedition of 1841 was its only attempt to take that territory. El Paso was only taken under Texas governance by Robert Neighbors in 1850, over four years after annexation.