Imagine if the Zimmermann Telegram had not been intercepted and cracked by Britain in 1917 and Mexico had joined the war against the Entente. The lack of American troops on the Western Front would have very little impact on the course of the war but, what about the Second World War? How would it be affected? This time line also has several other POD's which will be mentioned later.
First World War 1917-1918
The Zimmermann Telegram was a 1917 diplomatic proposal from the German Empire to Mexico to make war against the United States. The proposal was accepted by Mexico who later went on to invade the United States after she declared war in the May of 1917.
Sinking of the Mauretania
The passenger liner Mauretania (there were no Gallipoli landings in this time line) was sunk by German submarine U-20 (the same submarine which had sunk the Lusitania) on April 30th 1917, roughly 300 miles west of Ireland. There were 296 American passengers on board, all of whom perished in the sinking. The sinking eventually caused the U.S congress to declare war on Germany and her allies (excluding Mexico).
Entry of Mexico
The United States began sending many thousands of U.S soldiers to Europe (much faster than in OTL) and by July 1917 over 15,000 American soldiers were in or on their way to the trenches. On August 1st Mexico launched an invasion of the U.S states of California, Texas and New Mexico
Initial Mexican Invasion
Initial resistance to the Mexican invasion in New Mexico and California was light and by October, Las Vegas was under siege, San Diego and San Bernardino had fallen and a Battle was raging across Los Angeles. In Texas, however, resistance was much heavier, in comparison, although El Paso had fallen, Mexican troops had sustained heavy casualties.