The prospect of losing members as a consequence of Britain's vote to leave the EU represents a novel challenge for the bloc's leaders meeting in Brussels: they are more accustomed to grappling with expansion fatigue. Such exits don't amount to an existential threat to the EU, but they could further weaken a body that appears to be at war with itself.
The formal agenda includes debating the EU's trade policy and relations with Russia, but reform of the migration system is widely regarded as the most pressing - and contentious - issue.
For some members - most notably Poland, EU president Slovakia and Hungary - prevention of so-called Brexit contagion would start with scrapping the migration quota system and allowing members to control their own borders.
Others see reform of the bloc as the best defence against contagion, but differ widely on what reform means. Eurosceptics, at war with the Union, interpret reform to mean more rules from Brussels. For so-called federalists, reform would mean more unity and smoother cooperation in tackling continent-wide problems. For Eurosceptics, unity and cooperation are other words for federalism, an anathema.
Netherlands Prime Minister Mark Rutte addressed the dissension in the EU in an address on July 5, to the European Parliament: "It's not only British voters who have doubts about European cooperation. In other member states, too - including the Netherlands - there is widespread criticism and euroscepticism." He advises against rushing "blindly into more political integration and a more federal Europe."
Euractiv reports that French far right leader Marine Le Pen predicts that the EU would "die" if it went down the federalist path, while Belgian federalist MEP Guy Verhofstadt says it would die if it did not.
The Brussels summit will also pick up on decisions taken at the informal Bratislava summit in September, the first to wrestle with shaping the EU as a 27-member organization after Britain leaves. The UK hopes to maintain its trade relations with the EU while being free to say No at will to migrants. EU officials have told Britain that it will not be able to have an "a la carte" relationship with the EU if it wants access to the single market.
Date written/update: 2016-07-06