By Ken Bensinger | Contributor to The Christian Science Monitor

MEXICO CITY - Nearly a half-century ago, Mexico opened its arms to a young Cuban lawyer, a political exile who came here and began making big plans. And for decades, this nation always maintained a warm relationship with that lawyer - Fidel Castro.

But Mexico brought all that to a sudden close Sunday, cutting off relations with the bearded revolutionary's government and removing Mexico's ambassador to Havana.

The diplomatic equivalent of a knockout punch, thrown a day after Mr. Castro publicly questioned Mexico's sovereignty, was justified as a reaction to alleged Cuban meddling in Mexican political affairs in the wake of a bribery scandal here. But the move is only the latest in a string of events that have caused increasing tension and have, in just three years, laid waste to one of the world's strongest and oldest friendships.

That deterioration signals a significant change in the makeup of the alliances that define the hemisphere's political hegemony, paralleling the tenure of Vicente Fox as president of Mexico. Mr. Fox, who took power just a month before President Bush - and after 71 years of one-party rule - has traded in the contrarian's role Mexico held for most of the 20th century for a political agenda aligned with the United States and as a champion of democracy. And the closer Mexico gets to the US, the more it isolates Latin America's left-leaning states, like Brazil, Argentina, and Cuba, creating a bipolarity in the region, analysts say.

"The breakdown of relations [with Cuba] is simply a confirmation of the newfound influence that the US government has on Mexico," said Renato Davalos, a political columnist for the Mexico City newspaper, La Jornada.

Mexico yanked its ambassador, Roberta Lajous, a day after Mr. Castro suggested that Mexico's politics were being determined in Washington, not Mexico City. This came two weeks after Mexico, for the third straight year - and the third time ever - voted for a resolution condemning Cuba before the United Nations Human Rights Commission. The vote allowed the measure to pass, 22-21. Peru, which has also pulled its ambassador, voted for the resolution, while Argentina and Brazil abstained.

And last week, Cuba deported a Mexican businessman at the center of a bribery scandal who had fled to the island nation to escape prosecution. Havana announced that the businessman had confessed to participating in an international conspiracy to undermine the left-leaning political party whose leader is heading polls for the Mexican presidency in 2006.

Two years ago, Mexico cast its first-ever human rights vote against Cuba, prompting Castro to release an embarrassing tape recording of a conversation between he and Fox. The tape proved, despite Fox's previous denials, that the Mexican president had been asked by Mr. Bush to keep Castro out of a conference in Monterrey, Mexico, that they'd both be attending.

Six months later, Fox suddenly replaced his ambassador to Cuba, Richard Pascoe, who then said that he anticipated a collapse in relations between the two nations. Since then, Mr. Pascoe says, Mexico has been looking for pretexts to break off relations with Cuba. "It's a very unfortunate situation. I think it damages the geopolitical and geostrategic position of Mexico in Latin America," Pascoe told the Mexican press. "It demonstrates a country and a government that's too servile to the interests of Washington."

Mexico's foreign minister, Luis Ernesto Derbez, vigorously denied that idea, calling it "insulting." Nevertheless, despite schisms following the 9/11 attacks and the US's decision to go to war in Iraq, a political synchronicity has developed between the two countries under Fox and Bush.

Indeed, the White House declared - to the embarrassment of Mexico - that Mexico would be voting against Cuba before Fox had announced how he'd vote. Fox quickly said Mexico was undecided, then preceded to vote as the US said it would two days later.

And on Monday, just hours after Mexico pulled its ambassador, Secretary of State Colin Powell defended Mexico's move and vigorously criticized Castro, saying he presumed to "challenge free and independent nations that made their own choice to properly condemn him and his regime for its actions." This week Mr. Powell submitted a 500-page report to President Bush that suggests ways to unseat Castro.

While the Fox administration may embrace US political views, many Mexicans do not. Tuesday, some 2,000 Mexicans marched in the capital waving Cuban flags and calling for Fox's resignation. "Cuba, yes! Yankees, no!" they shouted. "The Mexican people love Cuba!"

It is precisely this kind of domestic political schism, analysts say, that has led to the rise of leftist movements throughout the region.

Argentina's close relationship with the US in the 1990s indirectly led to a economic collapse that brought left-leaning Nestor Kirchner to the nation's presidency, only months after Brazil elected Luíz Inácio Lula da Silva, a former union organizer. Each has worked to create a free trade area that doesn't include the US.

Ecuador, meanwhile, elected a leftist president last year, and in Bolivia the public overthrew a US-friendly president for the leader of a farmer's movement. In Venezuela - Cuba's strongest ally in the region and principal oil supplier - despite the huge push to unseat him (including a failed coup attempt hailed by the White House), President Hugo Chávez seems certain to win a recall vote.

Now with Mexico cutting ties to Cuba, the battle lines are more clearly drawn than ever before.

"Relations between the US and Latin America are at a dangerous low," says political analyst Andres Oppenheimer. "A return to the times of chaos and instability could convert itself into a serious problem for the national security of the United States."