"Preceding this period, and for some time thereafter, the western boundary of the United States was in doubt. Negotiations between the United States and Spain from 1803 until 1819 culminated in the Treaty of Amity, Settlement, and Limits, 1819, 8 Stat. 252. Under this treaty, the boundary "between the two countries" was in relevant part established along the west bank of the Sabine, 8 Stat. 254; the United States relinquished all of Texas [410 U.S. 702, 705] west of that boundary in exchange for Florida and the Spanish claim to the Oregon Territory; and it was provided that all islands in the Sabine belonged to the United States.
The United States renewed its efforts to acquire Texas, and when Mexico declared its independence from Spain in 1821, the United States began negotiating anew for the purchase of Texas. In the Treaty of Limits, 1828, 8 Stat. 372, the United States and Mexico recognized the boundary "between the two countries," id., at 374, on the west bank of the Sabine as established in the 1819 treaty with Spain. 2 Texas declared its independence from Mexico in 1836, 1 Laws, Republic of Texas, 3-7, in Gammel's Laws of Texas 1822-1897, was recognized as an independent nation by the United States in 1837, Cong. Globe, 24th Cong., 2

Sess., 83, 270, and in 1838 the Sabine boundary agreed upon with Spain in 1819, and with Mexico in 1828, was adopted by the United States and Texas, 8 Stat. 511. 3 The Sabine boundary remained unchanged when Texas was admitted as a State in 1845, 9 Stat. 108."
The Sabine boundary meaning the provocation by Taylor was an act of armed aggression against sovereign Mexican territory. Americans had long recognized the Sabine boundary, but Texas after 1836 decided they wanted to expand their claim to the Rio Grande, which the US didn't recognize until 1845.