It's probably the first time that events in Spain have decided the outcome of a Greek election. Last weekend the European Union agreed to loan Spain's nearly insolvent banks €100 billion on relatively easy terms. Syriza, the hard-left protest party that came from nowhere to dominate last month's election in Greece, will therefore almost certainly emerge from next Sunday's rerun of that election as the biggest party in parliament.
The party that wins the largest number of votes in a Greek election gets an extra 50 seats, so Syriza will probably lead the next Greek government. It would then demand a renegotiation of the EU's much harsher terms for bailing out the Greek economy — and it might even get it. That would prolong the agony of the euro, but it wouldn't actually save it. The common currency is doomed, at least in its current form, precisely because countries like Greece and Spain were allowed to join the euro.
It's not that they were more reckless and improvident than the Northern European countries who were really guaranteeing the common currency's value (though the Greeks certainly were). What dooms the euro is the fact that the Southern European economies are far less efficient.