For more than a century, the transatlantic partnership has been central to US foreign policy. The challenges we face — from terrorism to climate change to mass migration — extend far beyond national borders, so such cooperation is more important than ever.

Yet Trump’s ‘America First’ approach, together with his erratic leadership, is undermining the partnerships and the mutual agreements on which transatlantic — and, indeed, global — cooperation has long been based. Trump’s doctrine might please his core constituents, but it fails to account for even the most basic principles and mechanisms of international politics.

For all his supposed ‘deal-making’ skills, Trump seems not to understand that international agreements work only if they benefit all parties, and that this demands compromise. He is jeopardising the unity of the West, while bringing about negative, lasting change in the world order. Trump’s approach to defence, trade, and climate change is emblematic of this pattern.

A strong NATO is in the interests of both the US and the European Union. That is why Trump’s often-misleading criticisms of the alliance, which cast doubt on his loyalty to it, were so dangerous. Though Trump eventually endorsed Article 5 of the North Atlantic Treaty — the mutual-defence commitment that forms the core of NATO — the damage was done. As a result, the West is widely perceived — including by world leaders — to be divided and weak. Russian president, Vladimir Putin, for one, has taken this as a sign that he can continue to challenge the European and global security architecture.

In recent years, Putin has attempted to facilitate his violations of the sovereignty and territorial integrity of neighbouring countries by undermining the cohesion of the EU and NATO, whether through disinformation campaigns or by providing financial support to Euroskeptic and fascist groups in Europe. Trump’s equivocation about NATO has played directly into Putin’s hands.

The good news is that the EU seems to understand that, if it can’t rely on the US, it needs to take matters into its own hands, by pursuing more integrated security and defence policies. Last June, EU leaders agreed to activate ‘Permanent Structured Cooperation’ (PESCO), which allows the bloc to implement joint defence projects that strengthen its overall capabilities.

Cooperation among European armed forces must be improved. Collectively, European armies have more soldiers than the US and spend more on defence than Russia or China. But their efficiency is equivalent to just 10-15% that of the US. The lack of effective defence cooperation among EU member states costs up to €100bn annually. Given this, increasing cooperation could not be more important, though European defensive capabilities will be a complement to NATO, not a replacement.

Another policy that could undermine transatlantic security is Trump’s decertification of the Iran nuclear deal. Although the deal doesn’t address many aspects of Iran’s destabilising behaviour, especially its threats toward Israel, the EU — and the entire international community — remains convinced that the agreement is needed to enable constructive engagement with Iran. As for trade, Trump’s suspicion and rejection of international trade agreements has created a large political vacuum that others — especially China — will seek to fill. As Trump continues to tout his nationalist approach to trade, America’s partners are deepening their relationships with one another. The recent trade deal between the EU — which accounts for more world trade than China and the US combined — and Japan will be the world’s largest.

The EU must recognise that the US will not be as reliable a partner in the coming years as it has been since the end of WWII. Trump won’t be president forever, and the ties that bind the US and Europe will outlast him. But the EU needs to protect its own interests.

Elmar Brok is former chairman of the Committee on Foreign Affairs in the European Parliament and a senior official of the CDU Party in Germany.