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What if, in the Treaty of Guadalupe Hidalgo the annexed land from Mexico was all land north, east, and west of the Colorado and Rio Grande rivers? The US loses most of what's now Arizona, half of New Mexico, south-east Utah, and south-west Colorado, but gains Baja California and a almost-fully natural border with Mexico. How would this change anything?
This map is North America, right after the treaty in 1848. I tried my best with the river boundaries without tracing anything. Also, I'm too lazy to make Canada historically accurate.