As doors slam shut in Western Europe and razor-wire fences go up in the east, the massive wave of Middle Eastern refugees is backing up in the Balkans, where overwhelmed nations lack the will or resources to offer a long-term solution.

The ultimate destination for many of the refugees from Syria, Iraq, Libya and other war-torn nations is Germany, both for its rich benefits and its initial pledge to take in more than any other European Union nation. But when Germany announced that it was rescinding its offer and Hungary made itself a de facto firewall to Western Europe, tens of thousands of refugees already in transit were stranded in camps in Balkan nations including Croatia, Greece, Macedonia and Serbia.

"The Balkans should not become a parking lot or no man's land of blocked refugees,” said Johannes Hahn, a European Union commissioner from Austria.

"The Balkans should not become a parking lot or no man's land of blocked refugees,” said European commissioner Johannes Hahn.

- Johannes Hahn, Austrian official

Hahn called Germany’s withdrawn invitation a “big geopolitical mistake," and said it is not fair to blame Balkan nations for border control, whether too lax or too heavy-handed.

In Hungary, where refugees initially poured in from Serbia, officials closed borders last week with a massive fence and huge military presence. Hungarian Foreign Minister Peter Sijarto stoked Europe’s worst fears when he estimated that up to 35 million refugees could eventually attempt to migrate toward Europe, with thousands of terrorists hidden among them.

Germany's Federal Intelligence Service (BND) had a far more conservative estimate of 1.5 million refugees who will attempt to reach Western Europe via the so-called “Balkan route.” It is impossible to determine how many refugees have already reached European shores and are somewhere between Greece and Germany, but camps throughout the Balkans, as well as Western Europe, are packed with refugees in increasingly tense situations.

After Hungary took action, refugees streamed into Croatia from Serbia, prompting Croatia to close its border and trapping some 11,000 inside Serbia. Refugees were still working their way through Macedonia and into Serbia, where they remained stuck at the Hungarian border as Serbian and Hungarian officials traded terse statements.

Serbian Prime Minister Aleksandar Vucic accused Hungarian police of lobbing tear gas across the border and urged the EU to condemn the brutal behavior of the Hungarian police to refugees and media workers.

"Serbia itself can protect its territory and national dignity, and, if necessary, from the European Union, we will protect the European values,” Vucic said.

In Macedonia, where refugees reach before streaming into the former Yugoslavia, the government declared a state of emergency for the next year.

"Macedonia is considering building a fence on its border like he did Hungary, although it is not a long term solution," said Macedonian Foreign Minister Nikola Popovski

In Germany, where Chancellor Angela Merkel initially announced the country would take in 800,000 refugees this year, the head of the BND, Gerhard Schindler, said the most immediate danger comes from jihadists who are already in Germany.

"For the Jihadists, it is much easier to obtain a false or stolen passport and get on the plane. The real danger comes from Europe, jihadists returning to Germany and other European countries, once fought on the fronts of the Middle East,” Schindler said.

But German secret service officials have observed a disturbing new trend that combines the two threats. Radicals already in Germany are increasingly trying to penetrate the shelters that hold desperate and increasingly volatile refugees who made it to Germany.

"They have to be careful, because indigenous Islamists and Salafists could try to take advantage of the desperation of the refugees and to achieve their fanatical ideas," said Clemens Binninger, a member of Parliament.

Much of the fear has been driven by the fact that ISIS has clearly stated its plan to send jihadists to Europe amid the refugee wave. Fighters with valid documents from EU countries can enter and exit without being detected, and ISIS has reportedly seized hundreds of Syrian blank passports.

As a result, European jihadists could be returning amid the wave, and fighters from Syria and Iraq could be sneaking into Europe under humanitarian cover, where they can slip into society using doctored documents.

"Assassins of Paris, of Brussels and the train between Paris and Amsterdam were either radicalized European Muslims or European jihadists, returned from the battlefields,” Binninger said. “They're making contact with the network of political Salafists, which have been previously recruited to ISIS."

Turkey, a European Union member which has taken in an estimated 2 million refugees from Syria, blasted European nations for turning away refugees and making the continent a “Christian fortress,” in the words of Prime Minister Ahmet Davutoglu.