Migrants wait to leave a Macedonian registration and transit camp to board a train for Serbia last month. (Georgi Licovski/European Pressphoto Agency)

Europe this week effectively shut down the most popular land route to the heart of the continent, blocking the way for legions of desperate migrants from the Middle East and beyond. Over the past year, more than a million newcomers — including asylum seekers from countries such as war-torn Syria and Iraq as well as job seekers from other nations such as Pakistan and Bangladesh — have made the trip. They have washed ashore on the Greek islands, taken a ferry to Athens, and then traveled over land to countries such as Germany and Sweden that have lucrative job markets and offer generous benefits for those genuinely escaping war.

But actions taken this week by the European Union and several individual nations were a game-changer in Europe’s migrant crisis. Here’s what you need to know.

Who is impacted?

Migrants and asylum seekers trying to irregularly enter Europe’s core via the Balkans are now blocked. This includes job seekers as well as would-be refugees escaping wars in Syria, Iraq and Afghanistan.

So what happened?

A preliminary deal was struck between the E.U. and Turkey, the country where migrants and asylum seekers from across the developing world are hiring smugglers to enter Europe. The Turks would agree to take back all new migrants trying to cross the Aegean Sea. In exchange, Europe would agree to a more orderly program in which, for every Syrian picked up at sea and sent back, the E.U. takes in another Syrian directly from Turkey by air.  To discourage irregular crossings, any Syrian caught trying to traverse the Aegean would be immediately disqualified from the legal relocation program.

But right now, it looks as if only Syrians — and not Afghans and Iraqis, who make up almost half of all asylum seekers — would qualify for direct relocation. And the E.U.’s track record on actually accepting Syrians straight from third countries is dismal, prompting fears that most asylum seekers would simply get stuck in Turkey, a country where instability and violence are growing.

Europe's borders are slamming shut and leaders recently announced a preliminary deal under which everyone who crossed the sea to reach Greece would be sent back to Turkey. That hasn't stopped the flow of migrants streaming into Lesbos, yet. (Griff Witte,Jason Aldag/The Washington Post)

If the deal is preliminary, why is the route already closed?

Since the Turks haven’t yet started taking back migrants, many asylum seekers are still making for Greece — the entry point into Europe. But they are now stuck there because transit countries including Macedonia, Serbia, Slovenia and Croatia decided this week to suspend transit rights to the nationals of countries that require a visa to enter the E.U. That means that European citizens, Americans and valid visa holders from other nations can still enter. But migrants and asylum seekers without visas — which means almost all of them — cannot.

But isn’t it illegal under international law to deny asylum seekers a fair hearing?

Technically, yes. But transit nations such as Slovenia are getting around this by using a technicality. Any asylum seeker who makes it to its border and wants to claim official protection in Slovenia, for instance, may still do so. But almost all migrants are instead aiming for the wealthy European nations farther northwest. So, in other words, if they enter Slovenia, they have to stay there. And almost none of the asylum seekers want to do that.

But don't most countries in Europe now have open borders with passport-free travel between countries?

Some transit countries, including Macedonia and Serbia, are not part of the E.U. or the open-border zone known as the Schengen area that allows open transit among 26 nations from Portugal to Finland.

But technically, in the past, you could have hopped in a car in Slovenia and driven all the way to Lisbon without having to flash a passport. But those were the good old days.

Now, security threats from militants disguised as migrants and a general crackdown on asylum-seeker flows in recent weeks have led many European nations to reinstate border checks. They are likely to remain in place for the foreseeable future, at least until the migrant crisis completely subsides.

Is the route into Europe for migrants really closed?

Some migrants with enough cash are turning to smugglers to try to sneak through. Others are trying to cut through razor-wire fences and get past guards themselves. But that is an increasingly dangerous game in which migrants risk arrest, as well as violence by bandits and even the police in some countries.

Right now, most migrants who had hoped to transit through the Balkans are being blocked in Greece, and evidence suggests the route north is indeed closing. A key transit country, for instance, is Slovenia, a nation that in October had as many as 14,000 migrants transit each day. But since the new policy came into effect Wednesday, Slovenia’s foreign ministry has said that zero migrants have entered that country.

Can’t migrants take other routes into Europe, bypassing the countries barring the doors?

In southwestern Europe, that’s really hard. Hungary has already blocked the way and is arresting asylum seekers who try to cross. Albania and Bulgaria are possible options for the 13,000 migrants trapped at the Greek-Macedonian border. But the terrain in Albania — mountainous and wild — is hard to cross and riddled with local mafias extorting and beating travelers. In Bulgaria, the border is relatively well guarded, and authorities there have been arresting allegedly beating migrants who try that route.

Yet aid groups fear that the move to shut down the Balkan route could now lead more migrants to try the far riskier path to Europe known as the “Central Mediterranean route,” which runs through lawless Libya and a wider stretch of sea to Italy. That route was popular at the beginning of the current crisis in 2014. But thousands of deaths and deteriorating conditions in Libya led to more migrants taking the relatively easier route through Turkey, Greece and the Balkans. That could now change.