Donald Trump’s Oval Office tirade on Friday laid bare his instinct to harangue and bully those – even supposed allies such as Ukraine, fighting for its survival – who dare to disagree. Countries pushing global tax reform at the UN will be watching as US demands for subjugation play out in plain sight. His day-one threat to punish nations taxing US firms is an all-out attack on global fiscal cooperation. If multilateralism in taxation was already on shaky ground, Mr Trump’s return could bury it for good.

Under discussion is a new UN tax convention that may permit states to tax economic activity where it actually occurs, rather than allowing multinationals to shift profits to tax havens. The Tax Justice Network (TJN) said last year that nations lose $492bn (£390bn) annually due to corporate tax abuse. The global south bears the greatest losses, which undermine public services like health and education. If enacted, the convention would create a legally binding framework requiring multinationals to pay tax where they employ staff and do real business – not where they stash profits. This would replace the outdated arm’s-length principle with unitary taxation, ensuring fair profit allocation. It would mean an end to Amazon, Google and Apple putting billions through lower-tax jurisdictions while extracting wealth from higher-tax ones.

Before Mr Trump’s election, about half of global tax losses were facilitated by the eight nations opposed to a UN tax convention – Australia, Canada, Israel, Japan, New Zealand, South Korea, the UK and the US. Yet opposition takes two forms: constructive and destructive. When negotiations for the UN framework convention on international tax cooperation began last month, all participants committed to the convention’s principles except Mr Trump’s delegate, who walked out in defiance, calling on others to follow. The expected exodus never came. Washington was left isolated. Mr Trump’s “America first” became “America alone”.

But the US still has tremendous clout. As TJN’s new report, The International Tax Consequences of President Trump, highlights, talks among 120-plus nations on taxing cross-border digital services – led by the US-dominated OECD – are grinding towards a showdown. Mr Trump’s tariff threats against Canada and the EU are warning shots, aimed at countries daring to raise tax rates on multinationals, especially US ones. This fight isn’t just about taxation; it’s about sovereignty. Mr Trump’s administration is trying to strong-arm nations into preserving a system that shields corporate profits from fair taxation. The difference now is that the world is pushing back.

For decades, the US has had an unofficial veto over global tax rules, using its heft to shape – and then reject – OECD-led proposals. But this approach is no longer sustainable. The growing coalition behind the UN tax convention shows that many governments prefer to chart their own course. Mr Trump’s return forces a stark choice: stick with a broken system that fuels tax abuse or push forward without the US. Any attempt to tax multinationals fairly will face American retaliation, but clinging to the OECD’s US-dominated framework is a dead end.

A united front at the UN is needed to forge a global tax system not dictated by Washington’s whims. The cluster munitions convention succeeded without US involvement, proving international norms can shift without it. The world doesn’t need US approval to fix global taxation. It needs the will to move forward together.