SAN DIEGO - This seaside city was a bystander as liberal strongholds like San Francisco and Santa Cruz created identification cards for sick patients who use marijuana and wrote regulations to permit storefront pot dispensaries. Now, 10 years after Californians voted to decriminalize marijuana for medical purposes, conservative San Diego County is leading a backlash against the groundbreaking law. The county is challenging California's medical marijuana law in state court, saying it should not be required to comply with a state law directing counties to issue ID cards to users. At the same time, local police, working with federal agents,...