The problem is that to this very day nobody knows what the real debts of the banks are. Decades of deregulation and uncontrolled speculation in things like hedge funds, whose workings are very obscure, mean that the danger to the global financial system is systematically underestimated, like the bulk of an iceberg that cannot be seen because it is submerged.
What is known is that French and German banks are heavily exposed to Greece. This alone explains the tender concern with which the governments in Paris and Berlin view the Greek crisis. If (or rather, when) Greece defaults, it would be followed immediately by a crisis of the banking system in the two pivotal countries of the EU. That is why they cobbled together a “rescue fund”, the European Financial Stability Facility. But it is a case of too little and too late.
The crisis that began with the bankruptcy of banks has now moved on to express itself as the bankruptcy of whole nations. If Greece is allowed to collapse, other more important economies will follow. That is why the leaders of the Euro zone have called an emergency summit in Poland. Their previous plans are in ruins. The debt exchange that was agreed to in July is now dead in the water. They will have to throw it out and grant Greece some kind of debt relief to prevent a collapse that would have devastating effects throughout Europe.
http://www.marxist.com/the-greek-crisis-europe-on-the-brink-of-a-precipe.htm