The Obama administration on Friday issued new law-enforcement guidelines aimed at encouraging banks to start doing business with state-licensed marijuana suppliers, like those in Colorado, even though such enterprises remain illegal under federal law. Administration officials said the policy shift seeks to address problems faced by newly licensed recreational marijuana retailers in Colorado, and medical cannabis dispensaries in other states, that must operate on a cash-only basis, without access to financial services or credit. It remained to be seen whether many banks would feel sufficiently assured by the new policy, which the Justice and Treasury departments outlined in writing to...