The vast territory included in these two Territories was acquired
mostly from Mexico in 1848 as one of the terms of the treaty of peace
between the United States and that country made after the war of
1846-47. The war with Mexico was brought about by the refusal of the
Mexican Government to concede the claims of Texas to land between
the Rio Grande and the Nueces Rivers. The actual rights in the case
were somewhat obscure, but war was eagerly undertaken by the Southern
people, who believed that a further extension of slave territory
would be the ultimate result. The North was less enthusiastic, for
this reason, but sent a quota of troops into the field before whose
valor, directed by commanding officers who later became prominent in
the great war of the Rebellion, the Mexican armies were defeated. The
United States paid Mexico $15,000,000 for the territory ceded under
the treaty and in addition paid $3,500,000 in settlement of the claims
of private individuals. The boundary line remained in dispute for
five years more, until 1853, when James Gadsden negotiated a treaty
with Mexico settling all questions. Under its terms the United States
gained the Mesilla Valley, forming the southern part of what is now New
Mexico and Arizona, and comprising 20,000,000 acres. The United States
paid Mexico $10,000,000 for this land which was afterwards known as
the Gadsden purchase and is so marked on the larger maps issued by the
Interior Department at Washington. Including the territory acquired by
the Mexican war, the State of Texas and that included in the Gadsden
purchase, the whole area is sufficient to make one hundred and seventy
States the size of Connecticut.