The UN is our best defence against a third world war. As Trump wields the axe, who will fight to save it? | Simon Tisdall

If the US cuts off the cash it will have world-changing effects, but it's not the only country falling short in its obligations to the United Nations

The United Nations and its agencies have long struggled with funding shortfalls. Now an entrenched problem is becoming an acute crisis in the shadow of Donald Trump's executioner's axe. The US is the biggest contributor, at 22%, to the UN's core budget. In February, the White House announced a six-month review of US membership of all international organisations, conventions and treaties, including the UN, with a view to reducing or ending funding - and possible withdrawal. The deadline for decapitation falls next month.

Trump's abolition of the US Agency for International Development (USAID), and scrapping of most aid programmes, has already badly damaged UN-led and UN-backed humanitarian operations, which rely on discretionary funding. Yet Trump's axe symbolises a more fundamental threat - to multilateralism and the much-battered international rules-based order. The basic concept of collective responsibility for maintaining global peace and security, and collaboration in tackling shared problems - embodied by the UN since its creation 80 years ago last week - is on the chopping block.

Simon Tisdall is a Guardian columnist