Two old women, refugees, sit side by side in wheelchairs, their wrinkled faces staring at fresh coils of gleaming razor wire under two rows of fences that stretch across the farmland and into the mountains on either side. This barrier, its no man's land patrolled by rifle-wielding soldiers, lies between them and the many hundreds of kilometres they still have to roll under their wheels.

Sparrows flit through the fences and back, careless of the blades, chirping.

We are just outside the Greek border town of Idomeni, the choke point in the great European migration, one of the biggest movements of people since the Second World War.

Here the looming Balkan ranges funnel the flow of Syrian, Iraqi and Afghan refugees into a valley, and a wary Macedonia has in turn narrowed access to one sliding gate, wide enough for two to walk abreast.

More than half a million people passed through the Idomeni crossing between September and the end of November. In November more than 6000 people a day on average crossed the border: some days more than 10,000.

Today, perhaps none.

In the second half of 2015 the flow of refugees into Europe was a lively river, flicking around obstructions into new routes, past wave-through checkpoints. Now it is more like a painful, halting digestion, borders opening and closing in reaction to each other in a kind of peristalsis.

The gates close for any number of reasons: a landslide on a railway track, a protest by Macedonian taxi drivers accusing the government of depriving them of refugee cross-country fares.

But often the reason is simply this: there is no room for new arrivals until the last lot have moved again.

As soon as movement is cut it shunts up the line. Transit camps fill and the message passes back, Austria to Croatia to Serbia to Macedonia to Greece: stop the flow, we're full.

And now there are signs that these temporarily closed borders are about to become more permanent. The double-line fence is up, guarded and buttressed.

In late January the European Union gave Greece a three-month ultimatum to stop migrants crossing from Turkey, or else the EU would "quarantine" it outside the borderless Schengen area, ending free travel for more than just the refugees.

Macedonia is determined not to become the meat in this sandwich. This week, Macedonian officials told Deutsche Welle that "when the signal from the EU comes" it was ready to seal its southern border to refugees.

Jean-Claude Juncker, the head of the European Commission, promised to help: to "provide assistance [to Macedonia] to support controls on the border with Greece through the secondment of police/law enforcement officers and the provision of equipment".

The Greeks are not happy with this ultimatum, believing that Europe is washing its hands of the problem rather than helping. With some of their islands within swimming distance of Turkey, or a few hours in a dinghy, Interior Minister Nikos Toskas pointed out there was no practical way of stopping refugees from arriving "except via sinking or shooting".

A furious Greek Migration Minister Ioannis Mouzalas claimed a Belgian minister said "just push [refugees] back into the sea. The Belgian said 'go against the law. I do not care if it's illegal. Just push them back'."

He also accused Europe of short-changing Greece by providing smaller-than-promised numbers of everything from cots and fingerprinting machines to border guards.

But the crisis is not yet upon Greece, not today.

At 2.30pm in Idomeni, the border finally opens. For the rest of the afternoon, every half hour or so, 50-odd refugees are let through – as long as they can prove they are Syrian, Iraqi or Afghan, a condition imposed by Macedonia late last year.

It's enough: thanks to rough weather in the Aegean some days before, and farmers' protests blocking bus routes up from Piraeus, the number of refugees arriving at Idomeni today number in the hundreds, not thousands.

Don Johnston sits in the sun, and declares it a good day.

Johnston is the Red Cross' field co-ordinator for northern Greece. His is a 'surge' job – he swings in when the pressure's on, and helps the locals plans their humanitarian strategy.

Johnston was born in Colorado but is an adopted Aussie: for the last eight years he's lived in Melbourne and Sydney, jetting out for a few months each year to work for the Red Cross.

He's worked in just about all the instantly recognisable troublespots: Kurdistan, Iraq (he was there when Islamic State took Mosul), Liberia, the Haiti earthquake, South Sudan. "I hardly ever get deployed to Fiji," he jokes.

But he says this job is "singular" – a humanitarian crisis on an epic scale.

"I've never been involved in a mission like this," he says. "From a human point of view it's heartbreaking. What you are seeing is really difficult."

From one point of view his job is simple: when the refugees arrive in Idomeni they need food, shelter, clothing (it drops to freezing here at night), many need medical care (respiratory tract infections are common, almost every child has a cough). In concert with other groups such as the UNHCR and Doctors Without Borders, they do what they can.

But the biggest challenge is uncertainty, Johnston says. The situation can change in an instant. Just a few days earlier 80 buses arrived from Athens in one day.

"There were 7000 people here, the police in riot gear were super-tense, wouldn't let people move around," he says. "It was really cold. People were getting one or two meals a day. They are with their families, their kids.

"They're just regular folks. You talk to them, they're teachers, a carpenter, musicians. Just the entire population of a well functioning country is just being vacated and they're really uncertain about what's going to happen in their lives."

If the border closes for too long it can get tense. In early December, when it was closed for days without explanation, migrants rioted. Angry graffiti from the riot still colours many tents around the site.

"They really want information, that's the big thing," Johnston says. "They want to know what's going to happen with them, they want to know when the border's going to open, where things are at. There's a large desire. You can see they're just beset by uncertainty."

And it's hard on the local volunteers, too. Johnston says his people "tell me they cry every night when they go home".

Johnston says one of the contingencies he is planning for is Greece's nightmare: the borders close, but refugees keep coming.

This is the suspicion, too, of the UNHCR's head of the Idomeni field unit Alexandros Voulgaris. "[Idomeni] is a bottleneck," he says. "We could have a large population stranded here – hopefully the [Greek] authorities will get across this responsibility and move them to other facilities."

Greece is, indeed, setting up a series of "hotspot" camps to take refugee spillover. But locals have been protesting against them.

Outside one, on the outskirts of Thessaloniki, a group of protesters stand blocking the entrance beside their "Oxi" ("No") banners.

Theodore Papagriogoriou says, "our region is a poor region. If the European countries close their borders and we get 4000 people staying here, we can't provide jobs for these people, or houses, or food. This is a big problem for us.

"I don't' believe [the government] has a plan. That is our fear."

(One of the NGO officials comments to me: "I think he's right on the money").

And if the border closes, it may encourage the criminal people-smuggling trade.

According to reports, smugglers in the area sell fake ID papers at up to €1000 ($1600) each. And there are traffickers all over Idomeni, offering help to the nationalities who already aren't allowed over the border. They bundle into poorly ventilated trucks or risk precarious mountain tracks.

A nurse at the Idomeni camp told of a regular stream of patients coming back down from the mountains, beaten by the terrain – or beaten by Macedonian police.

Freelance photojournalist Nicola Zolin followed one group of "second-class refugees" just over a week ago. "Dark has come, time to go ... forest and smugglers the only choice," she tweeted. "Tonight, like every night, migrants from Pakistan, Iran, Maghreb will try their luck crossing [the] border by foot ... Stranded migrants still prefer to get smuggled through the 'mainstream' route than trying Bulgaria and Albania that they see as dangerous."

A few hours later she reported: "One hour walk to the border, then their smuggler got beaten by people with sticks. Now they go back to Athens."

Meanwhile, at the Idomeni camp, the refugees wait their turn.

Johnston says the demographics have changed this year – it used to be mostly young single males, now it is mostly families.

A Syrian woman is travelling with her three children: 10, 5 and 1½. Her home was destroyed, she says, and "if I stayed there for a million years we could not buy a house any more in Syria" – before the war her husband sold insurance, a concept now as alien as peace.

A young man, Shade, 21, fled Syria because he was told he had to join the army. He worked for eight months in Turkey to save money so he could catch a boat across to Greece – they crowded onto a small inflatable dinghy, "I was not scared for myself, death in Syria is the same to me as death in the water. I was just worried about the little ones on the boat."

And Naser Kasem, a 60-year-old Kurd from the north of Iraq, has a horror story: IS killed most of the people in his village, abducted his 22 year-old daughter, and killed two of his second daughter's young children.

He is bringing his wife and his third daughter to Europe, to safety. They floated over from Turkey on two inner tubes lashed together, with 13 other people, including a two-month-old baby.

"The water was coming through," he says. "We all covered the baby. I didn't care for myself, as long as the baby was alive."

Later, I see Kasem – and the baby – in the queue at the border (it's closed again).

There is movement on the other side. Macedonian President Gjorge Ivanov has come to visit.

He exchanges platitudes with the border guards.

But you can read a politician better by his actions than his words.

He doesn't greet the refugees, standing in a bemused line barely a metre away.

He doesn't even acknowledge them.

Instead, he goes to examine the double-row of buttressed fences, and the razor wire at their base, and he nods his approval.

To help refugees in Europe through the International Federation of Red Cross and Red Crescent Societies go to ifrc.org or donate to help Syrian refugees redcross.org.au

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