Are the twigs armed with sharp spines 1/2-2" long; the 4-9" pod does not split lengthwise along 2 lines? 

CLUE: this tree is native throughout the Southwest. 

It is a mesquite.

Are the twigs not armed with sharp spines, though some trees may have branched thorns on branches and the trunk; the 12-18" pod splits lengthwise along 2 lines? 

CLUE: this tree is not native to the western U.S. 

It is a honeylocust.