GARGOYLE: I'm really confused on your Spanish-American War description. The war involved the US and Spain. The war began after the American demand for Spain's peacefully resolving the Cuban fight for independence was rejected, though strong expansionist sentiment in the United States may have motivated the government to target Spain's remaining overseas territories: Cuba, Puerto Rico, the Philippines, Guam and the Caroline Islands. (see Wikipedia)
Mexico had been independent from Spain since 1821. While Mexico has had it's share of internal struggles for power since then, the US has not been an overt ally to any groups.
The US and Mexico fought their last war in 1847 which the US won, assuring US claim to portions of the North American southwest which allowed those areas to eventually apply for statehood in the US union. That war is not to be confused with the struggle some 12 years earlier by the Texans to oust Mexican control and briefly declare themselves the Republic of Texas. Because of the colorful figures of the Texas independence fight, such as Davy Crockett, Jim Bowie, etc., that "war" gets more folk song attention than does the true war in the next decade.
The US Mexican War in 1847 did indeed bring to the public eye a number of skilled soldiers who would later become either career military commanders or politicians, or both, Most of the West Point graduates who later commanded in the armies of the Union and Confederacy in 1861 cut their "war teeth" on the Mexican War. Others, such as Jefferson Davis, made names for themselves as military officers but moved into political circles afterward. Probably much more has been said (in song) on these due to their service in the War Between the States than anything they might have done in the Mexican War.