As I noted in a column last month, with President Trump promising to turn his back on the United States’ southern neighbor — renegotiating Nafta on presumably worse terms for Mexicobr and walling off the southwestern border — Mexican officials are desperately trying to convince his administration that the United States needs Mexico, too.br Ever since the skirmishing of the first few days of Donald J. Trump’s presidency, when President Enrique Peña Nieto of Mexico canceled a visit to the United States after Mr. Trump repeatedbr that Mexico would pay for his promised border wall, Mexican officials have hoped that more serene heads in the administration might moderate some of Mr. Trump’s more hostile proposals.br But as in 2005, the main buffer the United States has to stop Central American immigrantsbr from entering illegally across the border — its wall, as it were — is Mexico.br Two memos signed by Mr. Kelly — guidelines for Mr. Trump’s promised immigration crackdown — call for aggressively seeking out, detainingbr and deporting immigrants living without authorization in the United States.br Mexico Is It -br Just over 10 years ago, United States Border Patrol agents were startled by an unexpectedbr new development in their rear-guard battle to stop illegal immigration: Brazilians.br Fewer Mexicans are coming north across the border than are returning home from the United States.br And in a move sure to stir Mexican outrage, the guidelines direct immigration officials to return immigrants from any country caught entering illegallybr to “the territory of the foreign contiguous country from which they arrived” while they await removal proceedings in the United States.