BRUSSELS (REUTERS) - European Union leaders meeting in Brussels have a daunting list of long-running crises on their plates - notably migrants crowding in at their southern borders, Russia growling in the east, Britain's threat to quit and a desperate need to create jobs.
But Greece's looming bankruptcy next week forced its way on to a packed first-day agenda of the Brussels summit on Thursday, causing increasingly frustrated fellow leaders to again demand Prime Minister Alexis Tsipras stop asking them for favours and go and cut a cash-for-reform deal with their finance ministers.
As Athens lurched toward default, officials were obliged to keep reshuffling the timetable. One quipped it was no longer Britain for dinner but "Greece for dinner". Though in the end, Tsipras got an unscheduled two hours before the meal, pushing Cameron to a midnight slot after a heated debate on migration.
Plunged into crisis by the global crash in 2008 and unable to devalue its way out of debt due to the euro, Greece is also an element in the complex, interlinked challenges facing the EU - "Grexit", "Brexit", Russia and migration. One analyst this week dubbed them the "Four Horsemen" circling the summit.
At a meeting of conservative leaders that included German Chancellor Angela Merkel, frustration with the Greek leftists was clear. Antonio Lopez-Isturiz, the group's general secretary, said migration and Britain's threat to leave were being discussed "with whatever time we have left" after Greece.
Another conservative added: "There is a lot of frustration that this is using up an enormous amount of time and energy that should be dedicated to other very pressing issues."
Even a fix for Athens will not end much of Europe's trouble.
Greek economic problems are in part a version writ large of many in the euro zone, including other peripheral states bailed out in recent years but also Italy and even France.
The summit agenda includes discussion of new recommendations for making the euro zone more stable and dynamic and of efforts to take advantage of new digital services across an EU market of half a billion people.
But those will now get scant attention on Friday morning.
Economic drift in Europe and a fear of seeing Britain's economy being damaged by EU rules set for the German-led euro zone are among the reasons why Cameron has promised Britons a referendum by 2017 on quitting the bloc unless he can renegotiate terms.
So, too, is immigration, with open borders in Europe raising public concerns that link Africans and people from the Middle East risking their lives to cross the Mediterranean with migration to the west from the EU's poor, ex-Communist east.
Cameron did little more than formally launch his request for a renegotiation. Tusk spoke for many leaders in voicing a will to help London but warned that basic EU principles - notably free movement for workers - are not to be bargained with.
The migration issue, partly driven by unemployment in the south of Europe, partly by troubles in Libya and the Middle East following Western backing for the Arab Spring, also affects Greece. Tusk sought backing for a plan to distribute 40,000 Syrian and Eritrean refugees from Greece and Italy around other EU states, though many have already dismissed the proposal.
But hours of bad-tempered talks stalled.
This month has seen physical and diplomatic confrontations at the Italian-French border, at the Channel Tunnel linking France and Britain, and between Hungary and Austria as governments try to control the flow of migrants.
Some accuse Italy, and especially struggling Greece, of failing to monitor Mediterranean migrants arriving on their shores and instead turning a blind eye as they move on north.
One Greek minister last month threatened to "flood" Europe with migrants by ending attempts to stop them.
Others in Athens have cautioned that a bankrupt Greece could turn to its old Orthodox ally Moscow for help - worrying other Nato members, not least the United States. Washington also frets over its Atlantic partner Britain turning its back on Europe.
Having driven EU leaders into a fragile unity on trade sanctions that are hurting their own economies to punish Russia for its moves on Ukraine, Moscow is also complicating the EU's response to the migrant crisis by denying it U.N. support for more aggressive military action against Libyan people-smugglers.
EU governments rolled over sanctions this week until January, sparing the summit a discussion of Russia. But deep divisions remain between those who want a hard line, whatever the cost in terms of cold war, and others who fear backing Russia into an isolation that could raise risks of conflict.
Rem Korteweg of the Centre for European Reform said: "Four horsemen ... are circling together and affecting each other. "They raise issues that can only be solved if governments prioritise a European solution over narrow national agendas. If a European answer cannot be found, the horsemen will continue to promote chaos, instability and mutual recrimination."