We don't hear much about the Mexican-American war, and for good reason. Our "Manifest Destiny" was that the US should move all the way west to the Pacific Ocean. Mexican territory stood in the way. The US tried to buy it. Mexico refused to sell. So we moved in anyway, and when the Mexicans fought back, we declared war on them and finally "bought" a big slice of Texas, all of Arizona and New Mexico, and the state of California from them for fifteen million dollars. It didn't exactly buff our escutcheon.
Mexico itself in the 1830s was already an older country than the US, but exhausted by a revolution against Spanish rule and then by inner turmoil. Further, its northern colonies were distant from the capital, under-populated, and hard to supply and govern. The colony of Texas was poor and had intermittent trouble with Indians. So they did what seemed the sensible thing and invited rich Americans in to help settle the land. When there were enough immigrants, Mexico City, tried to put an end to the flood but they kept coming, displacing many of the natives in government positions and finally declaring the nation of Texas independent of Mexico, just as some politicians today speak of Texas seceding from the Union. Mexico sent an army under Santana to put a stop to this nonsense and the result was the Alamo, a defeat for the Texans, and then a battle that won Texas its brief independence from external rule.
One thing led to another. The US insisted that the international border be set at the Rio Grande instead of another river to the north, meaning more territory for the US. Mexico sent an army to oppose the movement and that's when President Polk declared war. The US won the war and we got all the territory we'd originally asked for, for the same amount of money.
At close to four hours, the documentary covers most of the bases. There are only a handful of talking historians and they don't intrude much. As in Ken Burns' iconic "Civil War," excerpts from letters are read. The narration is accurate enough, as far as a non-historian can tell. At least the speaker gets the pronunciation of Spanish place and family names correct. Reenactors are used from time to time, and contemporary painting and rough photographs.
Some things that are left out are minor. In the first World War, American GIs were called "doughboys." The name came from the first expedition to march south of the border in the Mexican-American war, when the soldiers were covered with dust the color of the adobe houses and were nicknamed "adobes." Some things that are left out are major considerations. Mexico at the time was ruled by an aristocracy, mostly of Spanish ancestry, that divided its social structure basically into two classes -- the wealthy patricians and the poorer peasants and craftsmen. It was based on the traditional European model and accepted as normal.
But slavery was outlawed in Mexico, whereas it was flourishing in the cotton-growing South, where so many American settlers came from. The Mexican government was primarily concerned with protecting its territory, but there was also a suspicion that the Americans wanted to conquer and spread slavery throughout Latin America and the Caribbean and, indeed, such plans were afoot in some circles of Congress and elsewhere.
In 1849, a lawyer named William Walker, a slavery enthusiast and a devout believer in Manifest Destiny, offered to take over the Mexican state of Sonora and fortify it as a buffer against Indian attacks. Mexico said no. So Walker and a handful of supporters tried to take it over anyway. They didn't get all of Sonora but they took over the sparsely populated area of Baja California and tried a repeat of the Republic of Texas. (Walker printed his own money, good in "the Republic of Sonora".) In California he was tried for a violation of the neutrality act and quickly acquitted. (It took eight minutes.) Later he established a slave republic in Nicaragua, with much popular backing from America's South and West. The British Navy arrested him, turned him over to the Central American governments, and they executed him.
That's a bit off the track, I know, but one of the problems with the film is that it doesn't closely examine the political and ethical dynamics of the war. We learn about suffering and battles. We don't hear much about resentment (if there is any) or slavery or even what President Polk THOUGHT about what another leader at a later time would have called "der Trieb nach Sud."
There was no real reason for the war, which is why we hear so little about it. But it did for our North American territory what the Spanish-American war did for our overseas ambitions. It's unfashionable, thank God, to do these sorts of things today, but 150 years ago the geopolitical situation was different -- lots of sub rosa intrigue backed up by the threat of brute force.