Question for history buffs about Texas?

The Republic-claimed borders were based upon the Treaties of Velasco between the newly created Texas Republic and General Santa Anna, who had been captured in battle. The eastern boundary had been defined by the Adams–Onís Treaty of 1819 between the United States and Spain, which recognized the Sabine River as the eastern boundary of Spanish Texas and western boundary of the Missouri Territory. Under the 1819 Adams–Onís Treaty, before Mexico's 1821 independence, the United States had renounced its claim to Spanish land to the east of the Rocky Mountains and to the north of the Rio Grande, which it claimed to have acquired as part of the Louisiana Purchase of 1803.

The republic's southern and western boundary with Mexico was disputed throughout the republic's existence, since Mexico disputed the independence of Texas. Texas claimed the Rio Grande as its southern boundary, while Mexico insisted that the Nueces River was the boundary. In practice, much of the disputed territory was occupied by the Comanche and outside the control of either state, but Texian claims included the eastern portions of New Mexico, which was administered by Mexico throughout this period.

The Mexican borders north of the Rio Grande were defined,  and the River itself formed the southern.

Back in those days, borders were changing frequently and were often disputed.

Ever heard of a map? They're not a new invention.