Yet the danger to Nato is not that it will be taken by storm or betrayed from within. It is that it will simply become irrelevant, like the picturesque ruined castles that dot the European landscape.
The row over Mr Claes symbolises the lack of political will that exists at the heart of Nato. If Europe had become the kind of place that no longer needed to have its security firmly underpinned, that would not matter; but with the threat of conflict ever present to the east and south, it is disturbing.
Mr Claes has become a symptom of political decay. For months his alleged involvement in a long-running Belgian scandal concerning the payment of bribes in connection with the purchase of military hardware has been the talk of Brussels. Augusta, the Italian helicopter company, was found to have made payments to Mr Claes's Flemish Socialist Party in return for aircraft orders when he was Belgian economics minister. It has been evident since Mr Claes first admitted that he has known that cash was offered in return for aircraft orders that something had to be done. Yet nobody has lifted a finger. And he will certainly not go on his own.
Now that the cumbersome Belgian legal system appears ready to put the cuffs on Mr Claes and take him downtown, the alliance's 16 nations will be forced to do something.
They have not acted before because they quite simply lacked the will to confront the many problems that are entailed in finding a new secretary- general.
Mr Claes himself only emerged from a grubby process of trading. He is there mainly because he's Belgian, and John Major would not allow another Belgian, Jean-Luc Dehaene, to hold Europe's other top job, President of the European Commission.
For similar reasons the allies have failed to get to grips with many of the strategic hot potatoes that have been dumped in Nato's lap. Nobody would pretend that the break-up of the Soviet Union could be handled easily, quickly or without argument in an alliance that is predicated on the existence of a hostile superpower on the doorstep. But the slow pace of change, the lack of imagination, the frequent reversals and side-stepping have become increasingly embarrassing.
A historical opportunity has been handed to those who make security policy in Europe and they have reacted with indecision and short-sightedness. For this, of course, everybody blames everybody else.
In Europe, the Americans are held up as the primary culprit. Washington has failed to exercise political leadership: it is charged with having vacillated between a policy of "Russia first" and Nato expansion, and then overplayed its hand in the Balkans. There is an element of truth in this. Bill Clinton's foreign policy has had more than a touch of Abstract Expressionism to it, with coherence established only in the past six months.
But the real problem lies in the European side of the alliance. The weak, divided and myopic political leadership that reigns in every European country is virtually incapable of coming to sound, long-term rational decisions. Fixated by economic weakness, riven by partisan political struggles and undermined by corruption scandals, European governments have floundered in the post-Cold War years. Mr Claes is fast becoming as potent a symbol of this political ineptitude as it is possible to be.
Nato was founded, and has lasted for 40 years, on the basis of a political equation that is paraphrased thus: to keep the Americans in, the Russians out and the Germans in check. This is self-evidently no longer a sustainable set of arguments.
Europe needs a serious debate about its security. If the Americans are to be kept in, then on what terms? How important is the US nuclear umbrella and, indeed, how important are nuclear weapons?
If the Russians are to be kept out, then how is the gap to be bridged between the West and Moscow? What will happen to states such as Ukraine? If the Germans are to be kept in check within new structures of European Union integration, how will the EU and Nato inter-relate? Can this be made to work while both organisations enlarge?
None of these questions has an easy answer, but there is precious little effort to find responses. Instead, the fate of Willy Claes and the choice of his successor are likely to occupy the time of people who would be better employed doing other things. The former Danish foreign minister, Uffe Ellemann-Jensen, is said to be the favourite. A reluctant Douglas Hurd is also said to have the backing of a number of European leaders, although his hands-off approach to the war in former Yugoslavia means he is unlikely to find favour in Washington.
As they seek a new name to pull out of the hat, European governments will inevitably confront the old and tired debate between Europeans and Atlanticists. It will all prove a diversion from the more profound existential questions that Nato ought to be facing up to. And the omens - Michael Portillo's Blackpool speech and a resurgent Gaullism in Paris - do not promise a quick or easy answer.
None of this is the fault of Mr Claes. Innocent or guilty of the charges of corruption, he is merely the wrong man in the wrong place at the wrong time. But every day that he remains in office is a day wasted. The quicker he goes and a successor is found, the quicker that Nato can move to deal with more important issues.Reuse content