what did texas believe in the boundary for the mexican war

Answer 1

Texas claimed the Rio Grande as its southern border. Mexico said the Nueces River to the north should be the border. President Polk pushed the whole issue to final resolution by forcing a showdown on January 13, 1846 by ordering 4,000 troops under the command of General Zachary Taylor to march from the Nueces River to the Rio Grande provocatively close to Mexican forces. Mexican forces attacked General Taylor in April. President Polk sent a war message to Congress in response.

Answer 2

Among its border claims, Texas chiefly believed that its border was at the Rio Grande River (which is now part of the current US/Texas - Mexico border) as this was what was established in the Treaties of Velasco(1836) which ended the conflict of the Texas Revolution between Mexico and itself.

As a result of this claim Texas held that all land between the Rio Grande River and the western part of the Arkansas River belonged to it.. In addition it held that several other adjacent territories then in Mexican possession were as well part of its natural territorial claim. After the Mexican-American War(1846-48), Texas gained most of its claims while ceding others to the US Federal Government.

Here is URL to an image illustrating Texas's border claims.

https://i.redd.it/1x1stsu7oft21.jpg