We're being sued by Mexico
No kidding. There's a case before the International Court of Justice entitled Avena and Other Mexican Nationals (Mexico v. United States of America).
It concerns Mexican citizens who face the death penalty in the U.S. Judgment will be rendered on March 31. Read the latest press release here, the first press release here, and the document index here.
This appears to be part of a larger attempt by Mexico to tie us up in litigation and force us to change our policies to fit their agenda. See, for instance, the earlier post "Mexico files human rights complaint against U.S. to the United Nations." This page suggests the possibility that Mexico might use the World Court to get compensation for their nationals who've died trying to get across our border. This CS Monitor report has some background.