(CNN) US President Donald Trump said Wednesday he "will absolutely do safe zones in Syria" for refugees fleeing violence in their country, devastated by years of civil war.

In his first televised interview as president, Trump said Germany and other European countries had made a tremendous mistake by allowing millions of refugees through their borders.

"I don't want that to happen here," he told ABC news. He did not give details as to what his safe zone plan for Syria entails.

What does Trump mean by safe zones?

Such zones are meant to be areas where civilians can live without fear of being targeted by any party in Syria's long civil war, protected by the international community. The Trump administration sees safe zones as the way to stem and even reverse the migration of Syrians to Europe and elsewhere.

So far, President Trump has not gone beyond the declaration that he "will absolutely do safe zones" for the Syrian people. It's an idea he broached in November 2015 as a candidate, when he proposed building "a big beautiful safe zone and you have whatever it is so people can live, and they'll be happier."

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The Department of Defense and the State Department are now charged with filling in the details. But the draft order from the White House instructs officials to "produce a plan to provide safe areas in Syria and in the surrounding region." The latter would be much less problematic.

Safe zones would need to be protected both by ground forces and the imposition of a no-fly zone and so require detailed planning and substantial resources to work. One big question: Would the administration seek the creation of safe zones through the United Nations, by agreement with other governments (principally Russia and Syria), or unilaterally? The last option would be extremely perilous -- given the Russian presence and the hostility of Syrian President Bashar al-Assad's regime -- and requires the injection of a substantial number of US military forces.

The Kremlin warned Thursday that "Washington must think about the potential consequences of establishing safe zones." Russia is perennially suspicious of Western plans to orchestrate regime change (think Libya, Serbia) disguised as humanitarian help.

How will the zones work logistically?

The size and location of such zones are critical; so is the "buy-in" (or lack of it) from the Syrian government, Russia and Turkey. If such a zone was established in northern Syria, for example, Turkish forces could be involved in protecting and supplying it. They and the Syrian groups they support have already carved out an area free of both ISIS and the Kurdish YPG militia. But many refugees would be very wary of returning to any part of Syria while the situation remains so unpredictable. It would be extraordinarily difficult and morally questionable to force them to do so.

Northern Syria has many fault lines: the Kurds, ISIS, the regime, other rebel factions including the former al Qaeda affiliate Jabhat Fateh al-Sham, all have a presence. And for ISIS -- even though it's on the defensive in much of northern Syria -- the presence of international peacekeepers trying to protect a safe zone would be a mouth-watering prospect.

The chaotic and fluid battlefield of Syria -- and the difficulty of protecting a large perimeter of open land -- makes the establishment of safe zones very hazardous. How would they be protected from infiltration by rebel groups?

For all these reasons, the Obama administration shied away from supporting safe zones, despite considerable lobbying from Turkey , where 2.8 million Syrian refugees are registered.

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There is also the issue of cost. In 2013, Gen. Martin Dempsey, chairman of the US Joint Chiefs of Staff, said a no-fly zone would "require hundreds of ground and sea-based aircraft, intelligence and electronic warfare support," costing up to $1 billion per month. Some of those resources would have to be diverted from missions against ISIS -- another prime objective of the administration. (A no-fly zone and a safe zone are not necessarily identical concepts, but given Assad's use of aerial attacks, a safe zone is difficult to imagine without a no-fly zone.)

President Trump has repeatedly suggested that the Gulf States would pay to maintain safe zones in Syria. He told the German newspaper Bild this month: "The Gulf states should have had to pay for them. After all, they have money like hardly anyone else has." But the Gulf states, which have backed different rebel factions, have offered no public support for such an idea. A year ago, Saudi Arabia talked about deploying ground forces to Syria to protect certain rebel-held areas from the regime, but the idea quickly evaporated as the battlefield swung in favor of Assad.

Have they been tried before? If so, what was the outcome?

Yes, in Bosnia, and the outcome was bad. In 1993, the international community designed six "safe havens" for Muslim communities, to protect them from Serb attacks. A UN report at the time said the goal included limiting "loss of life and property, deterring aggression, demonstrating international concern and involvement, setting the stage for political negotiations and facilitating the delivery of humanitarian aid." The objective in Syria would likely be very similar.

These safe havens were to be protected by a UN peacekeeping presence and, if necessary, air power. But the UN troops were inadequately armed and thin on the ground, and in 1995 the Bosnian Serb militia rolled into one of the safe havens -- Srebrenica -- and slaughtered thousands of men and boys.

The failure of the safe zone policy in Bosnia led NATO toward a full-fledged air war against the Bosnian Serbs.

There are fears that safe zones could promote division of Syria. Are these legitimate?

They are, depending on where they are and how permanent they become -- and how well they are administered. Rebels might want to take advantage of safe zones for resupply and sanctuary. Essentially, the Syrian government would have no role in such areas, which is why it has consistently opposed the idea as infringing on its sovereignty. The question is: At what point would it be safe to dismantle them? If they become sanctuaries for thousands who oppose the regime, do they end up being cantons of resistance?

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Who will benefit from such a plan, who will lose?

The trouble with safe zones is that they are a means to an end, not a permanent state. If -- and it's a big if -- there was international agreement on creating safe zones in Syria, and if Russia was able to bring the Assad regime on board, they could become a halfway house for the resettlement of at least some of the millions of Syrians languishing in refugee camps in neighboring countries. That would relieve some of the pressure on host governments such as Jordan, Lebanon and Turkey. Successful safe zones could also reduce the flow of Syrians to Europe.

Right now, there is no obvious incentive for either Assad or Russia to help create, or even tolerate, safe zones they don't control.

The question remains: What is the ultimate goal of US policy in Syria now? Does it still envisage the eventual removal of Assad and continued military support for the Kurds and moderate rebel factions (or those that survive)? Is it reduced simply to targeting ISIS? Does the US become "best supporting actor" to a process now co-owned by Russia, Turkey and Iran? The introduction of safe zones has to fit within a much larger scheme.