Over 25 years I have worked side by side with people committed to the fight against tyranny, extremism, and the ravages of war. These people, who daily risked their lives and those of their families in opposition to groups like al Qaeda, the Taliban, ISIS, Boko Haram, and al Shabab, are the greatest resource – and the most vulnerable – in these long struggles.

They help to run fragile governments trying to hold the peace; they guide US troops, diplomats, and aid workers; they jumpstart their economies creating employment and hope. These are the things on the ground in places like Iraq, Afghanistan and Somalia that ultimately make US citizens more safe at home.

And these committed partners are often targeted for their association with the United States and its allies. Their families are often forced to flee as refugees. Often too, when resettled, they seek educational opportunities that will help them rebuild their countries when they eventually return home.

These are precisely the people – our most important and most vulnerable partners – who are most harmed by President Trump's executive order on migration and refugees. The risk to them as individuals is extreme.

But the costs to the United States are even longer lasting. When we turn our most committed allies into disillusioned foes, when we appear to discriminate against the very people who risk their very lives for our shared ideals, we all become less safe.

Trump’s swift actions in his first week in office to stem immigration to the US were probably intended to deliver quick results. But I wonder if anyone in his entourage considered the substantial economic costs and related political risks that these measures entail.

In particular, has anybody sought to learn from Europe’s mistakes? Recent ODI research found that between 2014 and 2016 European countries committed to spend at least €1.7 billion to build walls and fortify borders in an attempt to reduce immigration.

Was this money well spent? Most likely not: while fewer people have attempted to cross the Mediterranean, the number of asylum requests is not falling as fast, as migrants and refugees attempt different and often more dangerous routes into Europe resulting in a record number of deaths.

Perhaps more importantly, the direct costs of border enforcement are only a small part of the potential waste: studies suggest that increased border control could cost the European economy between €470 billion and €1.4 trillion by 2025. For example, the increased checks and controls along the Oresund Bridge between Denmark and Sweden are reported to be costing the rail operator DSB €1.2 million a month in lost business.

Take a look at the scenes of chaos around US airports: the delays, cancellations and overall uncertainty that the bans will cause. What will this mean for the American economy? And if Trump cannot get the basic economics right, the nationalist politics are likely to quickly unravel.

Once a global leader on refugee protection, the United States is fast becoming its biggest threat. Trump’s executive ban on refugees and immigrants violates the spirit, if not the letter, of the 1951 Refugee Convention, which prohibits discrimination towards refugees and safeguards their right to protection.

The ban puts thousands of war victims’ lives in limbo and tears families apart, punishing the victims of some of the world’s most devastating humanitarian crises. 

What’s more, it will have ripple effects across the world, projecting the message that vague reference to ‘national security’ can justify closing your border to turn away people in need of protection.

Our research shows that increasing restrictions against refugees in Europe have opened up space for poorer countries – which already host the vast majority of the world’s refugees – to implement increasingly restrictive policies. Why, they ask, should they open their borders when richer countries are closing theirs?

We have seen this in Kenya’s threat to close Dadaab and send hundreds of thousands of refugees back to war-torn Somalia. Similarly, Jordan’s increasingly restrictive policies have left tens of thousands of refugees stranded at Jordan’s border with Syria. Trump’s executive order will only exacerbate these sentiments, putting more pressure on already overstretched economies in some of the world’s most volatile regions.

Ultimately, the people who will lose out most will be those who have fled their homes in search of protection. These refugees will be increasingly left with nowhere to turn and forced to resort to ever-riskier strategies as they struggle to reach safety.

Leaders worldwide must step up to the responsibilities abdicated by the new US administration. They must make clear to the world that Trump’s order is discriminatory and unacceptable, and that the values and obligations enshrined in the 1951 Refugee Convention remain valid regardless of America’s rapidly escalating domestic political crisis.

Trump’s immigration ban was remarkable, but in many ways the most striking thing has been the upwelling of public opposition, as citizens in the US and across the world have come together to protest the implementation of the executive order.

Over the weekend we have seen volunteer lawyers sitting on the floor at JFK airport drafting legal submissions, opponents of the order donating millions of dollars to the American Civil Liberties Union (ACLU), and tens of thousands of people gathering in protest at airports and cities across the US. Across the pond, a petition to oppose President Trump’s upcoming state visit to the UK gathered one million signatures in less than 24 hours.

In many ways, this represented a fundamentally human reaction to the contrast between the abstract rhetoric of ‘securing borders’ and the harsh reality of families torn apart, lives ruined and dreams shattered.

This is not the first time people have mobilised in response to punitive measures against refugees and migrants. Europe’s own response has been marked by a contrast between harsh government rhetoric and the rapid mobilisation of sections of the public determined to welcome refugees.

The challenge is now how to galvanise the more welcoming attitudes to refugees and migrants that exist in the US and across Europe. Far from being an aberration from the norm, these attitudes are built on our fundamental values, which in Europe are a product of the hatred that led to so much devastation in the past century.

Europe has learned the hard way how the language of hate can slowly be normalised and turn into something much more sinister. It is critical we heed the lessons of history and push back against populist rhetoric that will ultimately undermine our peace and security.