Meeting face-to-face with Mexican President Felipe Calderon, President Obama on Thursday said the U.S. is to blame for much of Mexico's drug violence, and he set up a major congressional gun-control battle by calling on the Senate to ratify a treaty designed to track and cut the flow of guns to other countries. Mr. Obama said he wants to renew a ban on some semiautomatic weapons but that it is not likely to pass Congress. Instead, he called for the Senate to ratify a decade-old hemispherewide treaty that would require nations to mark all weapons produced in the country and...