The bitterness between Mexico and the United States increased rapidly, and there was a feeling of war in the air long before war was actually declared. In these later days it is hard to find any good grounds for the war that followed, but Open hostilities began over the boundary line be tween the two republics, Mexico claiming that the Nueces River should mark the boundary, and our government sustaining the claim of Texas that the real boundary must be the Rio Grande (or Rio del Norte) River, about a hundred and twenty-five miles farther south -- Amazon.com