CARTAGENA, Colombia— President Barack Obama arrived Friday for a hemispheric summit hoping to talk up U.S. exports to the region, but he is being greeted by a subtle revolt against American antidrug policy and a host of complaints the White House would just as soon avoid.

The uprising on drug policy is led by some of Washington's closest and best-funded allies in Latin America, including Colombia, who are openly advocating for a discussion of legalization or decriminalization of the illegal drugs that have ravaged the region....