German and Greek government bonds are not guaranteed by any European authority, just as bonds issued by California or Arkansas are not guaranteed by the federal government or by the Federal Reserve.
And while the establishment of a permanent crisis resolution mechanism, the European Stability Mechanism (ESM), has created a backstop for national budgets, it is limited in size, conditional on adjustment programmes, and is an inter-governmental, not a federal, instrument. Sovereign debt in the euro area is thus exposed to credit risk in a way other advanced economies are not.