It's too soon to call it a rough patch, but a flap over cross-border trucking and unwelcome words about the drug war have led Mexico to push back against its powerful neighbor recently.
The trade dispute got tetchy last week when Mexico raised tariffs on scores of U.S. imports -- retaliation for Washington's decision to stop funding a program that allowed some Mexican trucks on U.S. highways under a free-trade agreement.
Days earlier, Mexican President Felipe Calderon and other top officials, annoyed by remarks suggesting that their government is losing turf to drug traffickers, let fly unusually sharp remarks about drug consumption and possible official corruption north of the border.
The developments hardly add up to a diplomatic row, and most analysts describe bilateral ties as fundamentally sound. But as Clinton makes her first visit to Latin America as secretary, the recent frictions highlight sensitivities that are always hovering around the relationship.
"Mexico is so entwined with us," said Peter Hakim, president of the Inter-American Dialogue, a think tank in Washington. "On almost any issue you'll find conflict and cooperation."
The visit by Clinton and one by President Obama next month will help set the tone for future dealings between Calderon and the new administration in Washington. Calderon and former President George W. Bush, both pro-business conservatives, were generally in sync.
Mexico's drug war and border security are taking on growing prominence in Washington because of violence that has killed more than 7,000 people in the last 15 months. U.S. lawmakers and security officials are increasingly concerned that the violence, which has included decapitations and bodies tossed into barrels of acid, could spill over the border.
Clinton's trip will take up the U.S.-Mexican efforts to fight the drug violence, but also will focus on the global economic crisis, trade and other issues.
She wants to show American support for Calderon's 2-year-old offensive against drug traffickers and, more broadly, to signal that the Obama administration intends to step up the U.S. anti-drug effort on its side of the border.
Obama has acknowledged U.S. responsibility as the primary supplier of money and firearms that sustain drug cartels in their fights against Mexican forces and one another.
"It's really a two-way situation there," Obama told reporters this month, promising a comprehensive border-security policy in coming months. "The drugs are coming north; we're sending funds and guns south."
Some analysts say the recent tough talk on the Mexican side may be aimed mainly at a domestic audience a few months before midterm congressional elections. Mexico's love-hate relationship with the United States means it can be politically profitable to keep the neighbor at arm's length.
Raul Benitez, an expert on U.S.-Mexico relations at the National Autonomous University of Mexico, said then-Mexican President Vicente Fox broke ranks with the United States by opposing the invasion of Iraq in 2003, the last midterm election year.
"This is exactly the same," Benitez said.
The drug issue has teased some of the underlying tensions to the surface. Mexican officials sounded irked recently when Dennis C. Blair, the U.S. national intelligence director, said Mexico was losing control of chunks of the country to drug cartels.
Mexican Interior Minister Fernando Gomez Mont said Blair's remarks didn't contribute to a climate of trust.
"There is not a single spot in the country" that is outside the government's authority, he said.
Calderon, not usually given to combative words toward the United States, called Blair's analysis "false, absurd" and said the U.S. needed to clean up its own act.
Many Mexicans blame the United States for the drug violence plaguing their country. Calderon and aides expressed frustration that the soaring death toll had been cited by commentators on the U.S. side to suggest that Mexico is losing and faces possible collapse.
The Calderon government contends that the violence is in fact a sign of success. The president says the killings can be attributed mainly to clashes among drug-trafficking gangs feeling pressured by the government's offensive.
Sergio Aguayo, a commentator and political analyst in Mexico City, said Calderon wants to make Mexico a top priority of the Obama administration.
"Calderon is testing Obama," Aguayo said. "The Mexican government is trying to let them know that . . . we are their neighbors and we count."
The trucking spat seems particularly knotty. Mexico accused the United States of violating the 1993 North American Free Trade Agreement this month when the Senate cut funding to the cross-border trucking program.
Calling the U.S. move "protectionist," Mexico slapped higher tariffs on U.S. goods as diverse as pears and dental floss. The list of products, from 40 U.S. states, was aimed at pressuring U.S. lawmakers by hitting as many districts as possible.
The Obama administration says it wants to craft a new trucking arrangement. Mexico has threatened to expand the list of U.S. goods facing higher tariffs at the border if the two sides can't reach a solution.
A senior State Department official said that U.S.-Mexican issues "never get that far from each country's nationalism." But the official, who spoke on condition of anonymity, said that the disagreements weren't enough to prevent collaboration.
Shannon K. O'Neil, a Latin America specialist at the New York-based Council on Foreign Relations, agreed: "Because of all the ties between our two countries, there will always be tensions. But there are and will be more issues where cooperation will benefit both countries more than isolation or unilateral actions."