In one sense, the euro is collapsing: dropping 10% against the dollar so far this year to around $1.282 (as of May 5, now only at 1.236) as European debt markets remain gripped by crisis. But in another sense, the euro seems almost impregnable: However much some policy makers, not least in Germany, might secretly wish to see Greece suspended or thrown out of the single currency, it is almost impossible for a country to leave. The solution to Europe's sovereign-debt crisis may ultimately require more unpalatable measures.

From a legal perspective, there is no mechanism to force a country out of the currency area, European Central Bank legal counsel Phoebus Athanassiou argued in a December 2009 working paper. And while the Lisbon Treaty introduced a means for states voluntarily to withdraw from the European Union, it was silent on leaving the euro. Ultimately, that means the only way a country could leave the euro would be to quit the EU, too, according to Mr. Athanassiou. That raises the stakes far higher, since it would affect the rights and obligations of citizens and companies.

Practically, too, leaving the euro would be extremely difficult. Beyond the huge logistical problems in introducing a new currency and untangling the national central bank from the Euro system, a euro exit followed by a devaluation would likely leave a country with a mountain of unserviceable euro-denominated debt, leading to major legal wrangles, mass personal bankruptcies and huge losses for creditors.

Nor would a weak economy like Greece gain much from currency devaluation in terms of increased trade and competitiveness, given its relatively low exports and manufacturing base. Exits from the euro could also damage one of the clear benefits the single currency has brought: a far deeper corporate-bond market that last year helped nonfinancial companies raise €277 billion ($360.2 billion) of cash, according to Société Générale.

Short of voters electing new isolationist governments ready to contemplate withdrawal from the EU, it looks as if euro-zone members are stuck with each other. That makes the likely endgame, should policy makers fail to halt the current crisis, a default within the euro zone, with the potential for support from other member states that would entail, no matter how unpalatable. All the more reason why member states need to draw up far tougher fiscal rules to ensure there can never be a repeat of this crisis.