The movement of tens of thousands of people toward Western Europe is overwhelming border authorities from Greece through the Balkans to Hungary and Slovenia.

Hungary's crackdown on its border with Serbia has set off a chain reaction. EU member Croatia is the latest to feel the squeeze and said Friday that it lacks the capacity to deal with more than 14,000 people who entered in just two days.

Nations have begun trading accusations about border obligations and other issues as Croatia transports people the long way round, from its border with Serbia to Hungary or further north to Slovenia.

Here are some questions and answers about the legal principles involved:

Q: Who gets to stay and who doesn't?

A: People arriving in Europe can apply for asylum or some other form of international protection if they are fleeing conflict such as the civil war in Syria and fear violence or death. Others, mostly economic migrants looking for work, are supposed to return home.

Q: What is the Dublin Regulation?

A: Under the so-called Dublin Regulation that governs the Schengen passport-free area, people wishing to apply for asylum must do so in the first member country they arrive in. In practice, Greece, the first landing point for many of those currently making their way through Europe, has been so overwhelmed by the almost 350,000 people who have arrived this year that it is unable to screen them.

Q: What is the Schengen passport-free area and how does it work?

A: The Schengen area is a group of 26 countries that have agreed to establish border controls, so you don't need a passport to travel between them once you're inside the area. Hungary and Slovenia are members, while Croatia, Serbia and Macedonia are not. Members of the zone are supposed to provide robust security at their borders with the outside world, in part by checking the passports of everyone entering their territory against a customs and criminal database. Schengen countries are only obliged to keep their external borders open during business hours. The way nations enforce those rules is their concern. Hungary, for example, insists that the razor-wire fence it has erected to keep migrants out entirely respects the laws because it has gates open during the day. EU partners argue that this does not reflect the spirit of the law.

Q: So what happens if someone applies for asylum in the first Schengen country he or she enters, then leaves before it's granted?

A: In theory, a person stopped in a Schengen area country other than the one where he or she first applied for asylum is supposed to be sent back to the first country, which should be Greece or Italy for most people currently traveling through Europe. In practice, it's a logistical nightmare, so it hasn't been happening much. Countries can decide not to apply the Dublin Convention and allow people to apply for asylum in their territory instead, which Germany has done for Syrian refugees.

Q: What about countries that aren't in the Schengen area?

A: Like all countries, non-Schengen nations Croatia, Serbia and Macedonia have their own visa regimes. Given the sheer numbers, these countries have been unable to enforce national visa laws, let alone register people. People who want to specifically apply for asylum there may do so, but most want to move on to countries like Germany and Sweden. As of Friday, for instance, only person out of the thousands who have been making their way through Croatia over the past few days had applied for asylum. The European Union has "readmission" agreements with Serbia and Macedonia, meaning that anyone arriving from there may be sent back at the request of an EU member state.

Q: Who gets sent home?

A: If a person does not qualify for asylum, he or she must return home, but at the moment, less than 40 percent of people are actually sent back. Given the scale of the migration challenge, the number of returns is likely to increase. Hungary says it aims to decide whether someone is eligible for asylum within eight to 10 days. The average time now is about nine months. Some people in Italy have been waiting two years for rulings on their applications.