Sigh ..... The 1836 Treaties of Velasco - they're online, so you can read them - which recognized Texas' independence defined the border between Mexico and Texas as the "Rio Grande del Norte". In violation of the treaties, Mexico subsequently claimed the border was the Nueces River, and consequently claimed about 2/3 of Texas (drawing an arbitrary line north from the origins of the Nueces). Mexican incursions into Texas' territory pre-dated Texas becoming a state, some by at least several years, but Texas lacked the ability to defend its treaty border.
About two weeks after Texas became a state, President Polk sent US troops to defend the Rio Grande border (January, 1846), in light of years of Mexican incursions into Republic of Texas territory and after the Mexican President claimed territory to the Sabine River - far more than previously claimed - basically all of Texas. Some 3 months later, a large (1500-2000) Mexican force crossed the Rio Grande into the US and killed or captured an 80-man US force that had been sent to scout reports of some sort of crossing. Unlike the Republic of Texas, the US had the means to defend its territory and did. This Mexican invasion of US territory and the "Thornton Affair" sparked the Mexican-American War.
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In 1519, Spain landed in the Aztec, Maya, Olmec lands of the New World.
In 1821, Spain departed and Mexico, which occupied Texas, declared its independence.
In 1836, The Treaties of Velasco were intended to end internal hostilities between
Mexico and Texas and NOT to confer independence on Texas.
As NEITHER treaty was ratified by the Mexican Govt, they had no force of law!!!
In 1845, Pres. Polk annexed Texas which triggered the Mexican War.
A binding Treaty exists when BOTH parties agree to its terms and conditions, Not one of them.
Try the Britannica.