Back in the early 1990s, when Los Angeles was stuck in the pits of a deep recession, somebody facetiously suggested that if Southern California were returned to Mexico _ maybe throw in south Texas and the San Joaquin Valley as well _ it could improve the economies of both countries. But given the current debate about immigration in all its economic and social complexity, that facetious idea left an increasingly compelling question: Where _ and what _ is the border? The obvious answer is that it's the line established by the Treaty of Guadalupe Hidalgo of 1848, which ended the...