Failure or war.
It sounds sensible enough: countries that use the same currency are all affected by the spending decisions others make. But such rules are only effective when two conditions are applied: (1) there is a shared norm of compliance; and (2) a credible enforcement mechanism is established. In other words, laws are obeyed within communities both because we generally share beliefs about their merit (murder should be punished), and because they are punished.
For these reasons, “international law” is the recourse of those who consider norms inadequate. But even international law relies on enforcement—in fact, enforcement is even more important internationally because the common societal basis of norms does not provide a foundation of compliance. There is obviously not a shared norm of thriftiness among EU countries; so the creditor states are attempting to impose enforcement.
Because there is no common norm for compliance, there has to be consequences for failing to comply. One ultimate consequence–the ultimate consequence on which all other enforcement actions must necessarily rest–is the forceful removal of those in power who fail to comply. Of course this is an enforcement action of last resort–but it is one that must necessarily be available to allow other enforcement actions to have teeth.
In the United States we’ve seen the Federal Bureau of Investigation arrest and detain individual government leaders for failure to obey the laws of the United States.
But in Europe, which has no sovereign federal government, but only a collection of cross-cooperating (and unelected) bureaucrats–to do the same thing looks awfully like a foreign nation using it’s troops to capture the leaders of another nation. That is, it looks a lot like an act of war.