BRUSSELS – When the architects of the euro started drawing up plans for its creation in the late 1980’s, economists warned them that a viable monetary union required more than an independent central bank and a framework for budgetary discipline. Study after study emphasized asymmetries within the future common-currency area, the possible inadequacy of a one-size-fits-all monetary policy, the weakness of adjustment channels in the absence of cross-border labor mobility, and the need for some sort of fiscal union involving insurance-type mechanisms to assist countries in trouble.