University of Oregon
Mapping History Project: Mexican Cession: Political Boundaries
Click on this map to see the boundaries of Mexico, Texas, and the United States before the Mexican Cession, and then click again to see the land ceded to the U.S. after the Treaty of Guadalupe Hidalgo. Be sure to click to see the key...
PBS
New Perspectives on the West: Annexation of Texas Joint Resolution
This site, which is provided for by the Public Broadcasting Service, gives the text of the Texas Joint Resolution, which made Texas part of the United States.
Curated OER
Perry Castaneda Map Collection: Admission of States and Territorial Acquisitions
Here's a map to show you when states were added to the Union. It also shows territories and when they were acquired.