There are several reasons why US President Donald Trump’s decision to withdraw American forces from northern Syria, and leave the region’s Kurds vulnerable to neighbouring Turkey’s military incursion, was a terrible one. The Kurdish forces in control of the region had been the principal US partner in the struggle against Islamic State. Trump’s abandonment of them reinforced already existing doubts in the region and around the world that the United States remains a reliable ally.

The decision also created conditions enabling hundreds, and potentially thousands, of IS terrorists in Kurdish-run prisons to go free—and presumably resume terrorist activities as soon as they’re given the opportunity. It’s more a question of when, not if, US forces will need to return to Syria to contend with a reconstituted IS (most likely without a local partner to bear the brunt of the fighting). In the meantime, the Kurds have turned to the Syrian government for protection against Turkish forces, a move that has allowed President Bashar al-Assad’s brutal regime (backed by Russia and Iran) to reassert its control over much of the country. For its part, the US has lost most of what leverage it had to influence a political outcome in Syria, despite the securing of a temporary halt ^[1] in the Turkish offensive.

Trump’s flawed decision seems to stem from his desire to make good on the promise he made during the 2016 election campaign to withdraw the US military from Syria and the Middle East more broadly. But this raises a larger question: given the negative impact of the move, why would he believe that it would prove popular at home?

One explanation is that Trump is confusing ‘endless wars’ with an open-ended military presence. This confusion is costly. What the US was doing in northern Syria was smart and efficient. Kurdish forces assumed the bulk of the combat role against IS; the US contribution was modest and largely confined to advising and providing intelligence support. Moreover, the US presence restrained the actions of the Turks, Syrians, Russians and Iranians. With the withdrawal of US troops, that restraint disappeared overnight.

More fundamentally, Trump’s decision taps into an old American tradition of isolationism, which has a lineage traceable to America’s founding fathers. It was in remission during the Cold War, but it has recently re-emerged, fuelled by the ‘intervention fatigue’ triggered by the long and expensive wars in Afghanistan and Iraq. It gains additional traction from the widespread view in the country that many domestic needs—from infrastructure to healthcare and education—are going unmet. A lack of emphasis on foreign policy and the world in US schools and media is also contributing to this inward turn.

Trump’s ‘America first’ slogan is premised on the idea that the costs of US world leadership far outweigh any benefits. The resources spent on activism abroad, according to this view, would be better spent at home.

However appealing such arguments may sound, the notion that the US can safely turn its back on the world and still thrive even as global order declines is seriously misguided. Trump has repeatedly claimed that Syria is not critical to America’s security, noting that it is thousands of miles away. But Americans learned the hard way on 11 September 2001 that distance is no guarantee of safety. Similarly, infectious disease, the effects of climate change and efforts to subvert elections don’t stop at national borders.

The costs of America’s global role are considerable by any measure. The defence budget alone now totals US$700 billion annually, and intelligence, foreign aid, diplomacy and maintaining a nuclear arsenal bring overall national-security spending to over US$800 billion. But as a percentage of GDP, this is well below the Cold War average. And history shows that the US economy nonetheless flourished even with this high level of spending.

To be sure, the US has many domestic shortcomings, from public education to healthcare, but for the most part these problems are not the result of a lack of spending. The country spends more than twice the OECD average on healthcare, but Americans don’t lead longer or healthier lives. Similarly, high spending on education doesn’t yield better results than in countries that spend less. How money is spent is always more important than how much is spent.

But such facts are nearly irrelevant when it comes to the political debate. Many of the candidates seeking the Democratic nomination to challenge Trump for the presidency in 2020 share at least some of his isolationist views, and opinion polls ^[2] reveal that many Americans do too. Trump is as much a reflection of America’s mood as its driver, and a certain degree of Trumpism—a desire to pull back from global commitments in general and military ones in particular—is likely to outlast the man.

At some point, things will change. History suggests that periods of retrenchment often end owing to some great geopolitical shock, followed by periods of exertion. The problem is that such shocks tend to be costly in terms of human lives and resources. But for now and the foreseeable future, the US is unlikely to conduct a foreign policy commensurate with its interests and strength.