Mexico has filed an amicus brief urging a federal court to invalidate SB1070, a controversial Arizona immigration law. The New York law firm Dewey & LeBoeuf filed the brief on behalf of Mexico. Mexico says that it seeks to protect the interest of its citizens who travel abroad and to prevent an erosion in US-Mexico relations. It its within the discretion of the judge whether or not to consider papers filed by nonparties.
This fairly routine development (filing an amicus brief) has become much more dramatic due to several misleading articles (like this one on Fox News), which state that Mexico has "joined" the litigation challenging the law or that Mexico has filed "a suit" against Arizona. These reports are misleading and untrue. An amicus brief is not a lawsuit. A party filing an amicus brief does not "join" the lawsuit.