For years, the Quadrilateral Security Dialogue - the so-called “Quad” linking the United States, Australia, India and Japan - has been presented as a cornerstone of Indo-Pacific stability.

The Quad was never a formal alliance though, but it did not need to be. Shared anxieties about China’s rise and continued aggression against Taiwan, coupled with a loosely aligned strategic vision, were usually enough to sustain it. That fragile coherence, however, now appears to be unravelling - and fast. All thanks to US President Donald Trump.

At the centre of the fracture lines lies Trump’s latest intervention in Iran, a move that started on February 28 and in the almost two months since, has sent shockwaves through diplomatic circles around the world. At the same time it has left Washington’s closest Indo-Pacific partners questioning whether the United States can be relied upon any more as a predictable strategic anchor.

The problem is not the substance of Trump’s Iran policy, however, but the very manner in which it is being executed. By acting with minimal group consultation and a now characteristic disregard for seemingly any and all allied sensitivities, the White House has time and again demonstrated a willingness to prioritise short-term, domestically driven calculations over its much longer-standing multilateral commitments. Day-to-day transactional decisions are now the norm for the US.

For the Quad, as an arrangement built on the back of mutual interests and trust, this is a potentially fatal flaw.

Because of this, Japan, as the most steadfast of Washington’s allies in the group, finds itself in an increasingly awkward position. Tokyo has invested heavily in the Quad over the years as a mechanism to balance China but without provoking outright confrontation. Trump’s actions in Iran though, have not gone down well in Japan, nor has his criticism of Tokyo's refusal to put itself in harm’s way by dragging the country into a broader geopolitical escalation so far removed from its own immediate security concerns.

India’s calculus is, if anything, even more delicate. New Delhi has long pursued a policy of strategic autonomy – critics at times may call it fence-sitting – carefully balancing its relationships with Washington, Moscow and Tehran. Trump’s Iran gambit cuts directly and unashamedly across that approach. By destabilising the region and heightening tensions with Tehran, the US has added a layer of confusion to India’s energy security - and fertiliser supply in particular - and undermines its broader diplomatic posture. For Indian officials, the message and realisation since late February is clear, and likely one they never wanted to face: alignment with Washington carries risks that may outweigh the benefits.

Australia, meanwhile, faces a familiar dilemma. As one of the United States’ security partners since WWII, Canberra has traditionally been willing to follow Washington’s lead – and back in the 1960s and 70s this included substantial military support in the Vietnam War.

Yet in Australia of 2026, there is growing unease about the unpredictability and self-centred modus-operandi of Trump’s current foreign policy. The Quad was meant to provide a framework for collective action in the Indo-Pacific. Instead it is being used by Trump as a vehicle behind which to draw allies into conflicts unrelated to the APAC region on the other side of the world. As many Australians see it, the more erratic Washington appears, the harder it becomes for Australian leaders to justify deepening or even continuing that alignment.

Taken together, these dynamics point to a broader erosion of confidence but not just in the Quad, in the United States as a whole. The European Union has already, for all intents and purposes broken away from the US. It is only a matter of time before the Quad either ceases to function or decides to go its own way, without the US. In Washington and around the world, Trump’s critics have long argued that his approach to foreign policy undermines the very alliances that underpin American power and the latest developments in Iran lend considerable weight to that argument.

The comparison with Nato is striking too. While the transatlantic alliance remains intact – barely – it has repeatedly been strained by Trump’s rhetoric and actions. Questions about burden-sharing, collective defence and the reliability of American commitments when the US president constantly criticises his allies have become routine.

And now, the Quad, lacking Nato’s institutional depth and formal obligations, is vulnerable to similar pressures. If the trust on which the group exists begins to fray, there is little to hold it together.

This in turn creates opportunities for others – namely China.

Beijing, which has always viewed the Quad with suspicion, will be watching closely for signs of disunity and with moves against Taiwan, and a drawn out stare-down with Tokyo over regional claims in the East and South China Seas has already moved to demonstrate how the US will not come to the aid of its so-called Quad allies.

None of this is to suggest that the Quad is beyond repair though. Strategic interests among its members still overlap, and concerns about China are unlikely to disappear. But rebuilding trust lost by US actions over Iran will not be straightforward, and under President Donald Trump is more than likely unachievable.