"The euro zone developed a balance-of-payments problem; some of the countries in the single currency accumulated...

"The euro zone developed a balance-of-payments problem; some of the countries in the single currency accumulated large external debts. To service those debts, the deficit countries need to become surplus countries, which is difficult to do without the flexibility of a floating currency. The difficulty of adjustment has led markets to doubt the solvency of some institutions, and these doubts have, in the absence of a lender-of-last-resort, metastasised into a contagion that threatens to leave banks and sovereigns bankrupt."
http://www.economist.com/blogs/freeexchange/2011/11/euro-crisis-21