In a gesture of reconciliation a century and a half after the neighbours' three-year war, US President Bill Clinton laid a wreath on Tuesday at the castle site known as the Ninos Heroes (Boy Heros) monument.
The 1847 battle, and the Mexican-American war in general - in which 50,000 Mexicans died and the nation lost what are now Southern California, Arizona, New Mexico, Texas and parts of Nevada, Utah and Colorado - left a deep wound on the Mexican psyche.
That was part of the reason several hundred Mexicans protested Mr Clinton's first official visit to his southern neighbour, burning an American flag and yelling "Yankee go home!" More specifically, they were angered by Mr Clinton's support for President Ernesto Zedillo's Institutional Revolutionary Party (PRI), a long-ruling monster most resembling the old Soviet communist party but with little discernible ideology other than to cling to power.
Many Mexicans blame successive US governments for allowing the PRI to retain power for almost seven decades, traditionally through fraud, in the interests of regional stability but at the expense of democracy and human rights.
In a report last week, the US-based group Human Rights Watch/Americas slammed the Mexican government and the PRI for what it called widespread violence against peasants and political opponents throughout southern Mexico.
Also last week, 12 European tourists were expelled from Mexico simply for following a colourful anti-government protest march by Chol Indians in the state of Chiapas.
Mr Clinton attempted to offset criticism of pro-PRI bias through historic meetings - no US president had ever done so - with two opposition leaders from both the left and the right of the PRI. The two parties, the catholic and conservative National Action Party (PAN) and the social-democratic Party of the Democratic Revolution (PRD) have gained ground in recent years as outside pressure forced the PRI to jettison, slowly and reluctantly, some of its old fraudulent habits and edge at least in the direction of democracy.
But Mexican analysts noted that Mr Clinton gave only 15 minutes each to the opposition leaders and said the PRI's continuing control of much of the press - including through cash payments and perks to political reporters - would ensure that Mr Zedillo would benefit most from Mr Clinton's visit, his first as President to any Latin American nation.
It was no coincidence, those analysts say, that Mr Zedillo wanted the US President in town before crucial congressional and local elections in July. For the first time, pundits are predicting the PRI could lose its congressional majority. Also at stake is the new, elective post of Governor of Mexico City - replacing the old system of a hand-picked (by the ruling party) mayor - seen likely to be the second most powerful figure in the country.
Mr Clinton and Mr Zedillo signed several agreements but they were described variously by diplomats "symbolic", "modest" or "small-bore". There was no sign of progress on the key issues of drug trafficking through Mexico, Mexican anger at tougher US immigration laws or whether US anti-narcotics agents can carry weapons in Mexico.
Mr Clinton was yesterday moving on to San Jose, Costa Rica, for a summit with Central American leaders, followed by a Caribbean summit in Barbados at the weekend.