BRUSSELS — European leaders lashed out Sunday at each other’s handling of the continent’s greatest immigration crisis since World War II, even as they came together to seek ways to ease the plight of the tens of thousands marching across the Balkans toward the European Union’s heartland. At an emergency summit in Brussels, 11 European Union and Balkan leaders were especially looking to shore up Greece’s porous border with Turkey and slow the flow of people heading north toward the European Union’s heartland. Slovenian Prime Minister Miro Cerar said his tiny Alpine nation was being overwhelmed by the refugees — with 60,000 arriving in the last 10 days — and was not receiving enough help from its EU partners. Serbian Prime Minister Aleksandar Vucic asked a fundamental question that the 28-nation bloc and non-EU nations like Serbia have been unable to answer since the migratory trek across the Mediterranean and through Turkey started last spring: “What we are going to do with hundreds of thousands of these people?”