us won the war with mexico
Southern Arizona and New Mexico, specifically the Mesilla Valley. This transaction is known as the Gadsden Purchase (1853).
The Zimmerman Telegram.
Only if you return by land; if you take a flight you must have your passport with you.
The war with Mexico changed the US border and land holdings significantly. The US was advantaged as it acquired New Mexico to be part of United States.
Mexico and Canada.
New Mexico.
Take a passport. As a US citizen, you won't have much trouble entering Mexico, but you will have to establish your citizenship to get back.
To fly into Mexico you HAVE to have a passport. As of right now the only way you can get into Mexico without a passport is by land travel, but that will change as of Summer 2009. As of Summer 2009 you will need a passport for any land travel into Mexico and Canada.
Mexico and Canada
there isn't a single answer but here is a couple of reasons: 1)we intercepted a letter to Mexico from Germany trying to persuade Mexico to attack US and Germany would return TX and other land we took from Mexico. 2)we knew if Britain fell against Germany that they would come after us next 3)Germany attack unarmed US supply ships with their U-Boats and us civil non supply and non military boats
Germany. More specifically, the German ambassador in Mexico, Arthur Zimmerman.