Viritually all EU member states had vehemently opposed any treaty change going into the summit, but in the end they were convinced by Germany's need for the change in order to avoid a legal clash with its Karlsruhe-based Constitutional Court.
The leaders agreed to construct a permanent crisis mechanism to fill the void left when the existing but temporary €110 billion bail-out package for Greece and €440 billion fund set up for the eurozone as a whole expire in 2013.
According to diplomats, it is currently unclear whether this new mechanism would involve participation of eurozone members alone or the full 27 EU member states, including those who do not use the euro.
Germany is worried that any permanent structure could run afoul of treaty rules forbidding EU bail-outs of member states and be struck down by the country's strict Constitutional Court, thus opening the euro once again to an assault by markets as occurred in the spring.
Caught between the need for a structural change and their fear of both the activism of Karlsruhe and the growing euroscepticism of citizens, the other leaders signed off on the move only so long as the change envisaged was "small, small, small - the smallest possible ... in order to ensure there is no possibility of referendums," in the words of a Danish diplomat speaking to EUobserver.
The method EU leaders chose to achieve the change will be via what is called the "special revision procedure," introduced by the Lisbon Treaty, under which the treaty can be amended by the European Council alone, so long as there is unanimity and the changes do not extend the competences of the European Union.