From our colleagues Cooley, Richardson, and Schoenholtz:
Policymakers in the eurozone underestimate how difficult it will be to hold the remaining euro area together if Greece exits. … The reason is that the euro area was designed to be irreversible, [there is] no provision for exit. A Greek exit would be a logistical nightmare. [But it would make] the commitment of other members not to exit vastly more difficult to make credible, [and therefore] subject to doubts and speculative attacks.