DEL RIO, Texas - Standing in a cramped federal courtroom last month, illegal immigrant Walter Oscar Portillo-Machado pleaded with a judge for mercy. But he came to the wrong place for that. The Salvadoran man was caught along a 210-mile stretch of the Texas-Mexico border that has been set up as zero-tolerance zone for illegal immigration. Instead of merely getting sent back home, immigrants here are arrested, prosecuted, and sometimes sentenced to prison before they are formally kicked out of the country. The effort began late last year along a border area that includes the Rio Grande border towns of...