Alexis Tsipras risks leading Greece to disaster—and he has just days to pull the country back from the brink.

It is three months since Mr. Tsipras forced a snap election and six weeks since he became prime minister. In that time, the Greek economy has nose-dived. A tentative recovery has been stopped in its tracks: Tax receipts fell more than €1 billion ($1.05 billion) below target in December and January, the banks have been emptied of more than €20 billion of deposits, loan defaults have risen and investment has been put...