8 4 6 : ▁Mexican – American ▁War ▁declared , ▁ch . ▁ 1 6 , ▁▁ ▁July ▁ 9 , ▁ 1 8 4 6 : ▁District ▁of ▁Columbia ▁retro cess ion , ▁ch . ▁ 3 5 , ▁▁ ▁July ▁ 3 0 , ▁ 1 8 4 6 : ▁Walker ▁tar iff , ▁ch . ▁ 7 4 , ▁ ▁Tre aties ▁ ▁June ▁ 1 5 , ▁ 1 8 4 6 : ▁Oregon ▁Tre aty ▁established ▁the ▁ 4 9 th ▁parallel ▁as ▁the ▁border ▁between ▁the ▁United ▁States ▁and ▁Canada , ▁from ▁the ▁Rock y ▁Mountains ▁to ▁the ▁Stra it ▁of ▁Juan ▁de ▁Fu ca ▁ ▁January ▁ 1 3 , ▁ 1 8 4 7 : ▁Tre aty ▁of ▁C ahu eng a ▁ended ▁the ▁fighting ▁in ▁the ▁Mexican – American ▁War ▁in ▁California ▁( not ▁a ▁formal ▁treat y ▁between ▁nations ▁but ▁an ▁inform al ▁agreement ▁between ▁rival ▁military ▁forces ) ▁ ▁States ▁admitted ▁ ▁December ▁ 2 9 , ▁ 1 8 4 5 : ▁Texas ▁admitted ▁as ▁the ▁ 2 8 th ▁state ▁ ▁December ▁ 2 8 , ▁ 1 8 4 6 : ▁Iowa ▁admitted ▁as ▁the ▁ 2 9 th ▁state ▁ ▁Party ▁summary ▁ ▁Senate ▁ ▁During ▁this ▁con gress , ▁two ▁Senate ▁seats ▁were ▁added ▁for ▁each ▁of ▁the ▁new ▁states ▁of ▁Texas ▁and ▁Iowa . ▁ ▁House ▁of ▁Representatives ▁During ▁this ▁con gress , ▁two ▁House ▁seats ▁were ▁added ▁for ▁each ▁of ▁the ▁new ▁states ▁of ▁Texas