Yes, you are correct!!
James K Polk wanted more land, and asked Mexico if they could give them money for the land. Mexico though was like "No." Getting angry, he moved troops into a disputed Mexican area. The tension grew more and more. Texas became with the states on Dec. 29th 1845. The united states inherited the southern and western border dispute. Now Mexico had refused to recognize Texas's independence or the US offers for their territory. Thus, turning into a war.