The Mexican American Wars Yahoo Group has a set of free miniature wargames rules for the Mexican War called Across The Rio Grande. The authors write:
Across the Rio Grande is a simple, fast-playing, battalion-level rules set that is intended to impart the flavor of the Mexican-American War and simulate the battles fought during that period. By its nature it was a war of small unit actions where one man could turn a battle around through a heroic action or foolish blunder. Morale was as important as anything else for keeping an army together, and it plays a big part in these rules. Weaponry didn�t vary enough between the units for it to have a great impact on a battle, with the exception of the American light artillery units, which on numerous occasions had a disproportionate influence on a battle, and this has been built into the rules. Finally, hand-to-hand combat has probably been overemphasized in contemporary art and text since it didn�t actually happen that often. This is reflected in the rules for melee combat which result in few actual casualties, but which have a great effect on unit morale, especially for the losers. The intent of this set of rules is to be a quick-to-learn and fun game especially for gamers unfamiliar with the Mexican-American War and those who prefer relatively simple rules systems.