MEXICO CITY - It has been a trying week for Mexico-U.S. relations: a tense border confrontation between U.S. agents and apparent drug traffickers, a Mexican group's offer to print maps of the Arizona desert for illegal migrants and an exchange of terse diplomatic notes. The administration of Mexican President Vicente Fox has its share of quarrels with other countries, but this promised to be one of the trickiest — involving the country's northern neighbor and largest trading partner at a time when the U.S. Congress is debating immigration reform. For Mexico, migration to the United States is a mainstay of...