European unity about open borders has fractured over the refugee crisis and nowhere is this meltdown more evident than on Hungary's borders with Serbia and Croatia.

Hungary and Croatia are trading threats and tensions along their border are rising as desperate refugees try to physically push their way into Hungary. The Hungarians have built a wall along its border with Serbia, but that hasn't slowed the human tide threatening to engulf eastern Europe.

Reuters:

More than 20,000 migrants, many of them refugees from the Syrian war, have trekked into Croatia since Tuesday, when Hungary used a metal fence, tear gas and water cannon on its southern border with Serbia to bar their route into the European Union.

EU leaders, deeply divided, are due to meet on Wednesday in a fresh attempt to agree on how and where to distribute 160,000 refugees among their countries, but the noises from some of the newer members of the bloc were far from friendly.

Hungary, where the right-wing government of Viktor Orban has vowed to defend “Christian Europe” against the mainly Muslim migrants, accused Croatia of "violating Hungary's sovereignty" by sending buses and trains packed with migrants over their joint border. It warned it might block Zagreb’s accession to Europe’s Schengen zone of passport-free travel.

"Croatia's government has continuously lied in the face of Hungarians, Croatians, of the EU and its citizens," Foreign Minister Peter Szijjarto told a news conference. "What kind of European solidarity is this?"

Croatian Prime Minister Zoran Milanovic said that, unlike Hungary, he would not use “brute force” to keep people out, nor would his government make them stay against their will. The buses and trains would keep running to Hungary, he said.

“We forced them (to accept the migrants), by sending people up there. And we’ll keep doing it,” he told reporters.

There was a bottleneck forming in Hungary because the refugees couldn't get to their ultimate destination - the rich countries of northern Europe. Austria, Germany, Italy, and France had all closed their boders - temporarily they say - until they can sort out who is going to take how many refugees. 

So the refugees were stuck in Hungary while Orban tried to staunch the flow coming from Croatia. Then on Saturday, Hungary finally relented and allowed transit by the refugees through their country as Austria was forced to reopen its borders:

Austrian police said more than 11,000 entered the country on Saturday, and another 4,700 arrived Sunday through the Nickelsdorf border post from Hungary.

Most of the migrants had made the gruelling journey across the Balkans into western Europe, with Croatia saying 21,000 had entered its territory in the past four days.

The Austrian Red Cross said 2,000 made it into Germany by avoiding border posts.

As long as Austria and Germany open their arms to accept the refugees - regardless of their number - people will keep coming. But the nations of southern and eastern Europe must pay the price for German generosity by facilitating their movements across their borders. It hasn't reached the point of a shooting war yet. But Orban is sounding more and more bellicose toward his neighbors and as the crisis continues, it wouldn't take much to set of an incident that would result in a tragic military confrontation.