Donald Tusk

Around half a million people have fled to Europe so far this year in search of sanctuary or jobs. As numbers swell, nations have tightened border security. Hungary has a razor-wire fence along its border with Serbia.

Countries in eastern Europe with little recent experience of migration are trading barbs over who is to blame, while Balkans neighbors Serbia and Croatia — who were at war in the early 1990s — issued threats and ultimatums.

Syriza's victory in Sunday's election marks a personal triumph for Tsipras, who served as prime minister between January and August — a tumultuous period that saw Greece's future in the 19-country eurozone come under real threat and strict banking controls imposed.

At 41, Tsipras dominates Greek politics despite a major policy U-turn that saw him go against the anti-austerity platform that swept him to power in elections in January in return for a multibillion bailout that keeps Greece in the eurozone.

Overcoming their surprise when Tsipras failed to present them with a detailed plan, the leaders reluctantly agreed to a final summit Sunday, saying that could give both sides an opportunity to stave off collapse of the struggling but defiant member nation.

Underscoring the gravity of the challenge, European Union President Donald Tusk decided to call all 28 EU leaders to Brussels instead of only the 19 eurozone members, because, for the bloc, it "is maybe the most critical moment in our history."