Leading article: This quarrel is, above all, over a simple issue of trust

There are times when it is right to be thankful for small mercies. With security in Iraq and Afghanistan deteriorating and the proposed UN force for southern Lebanon taking far longer to be formed than had been hoped, the restraint that has surrounded Iran's delivery of its 21-page missive this week gives cause for just a little hope.

Certainly, Iran has been playing a canny diplomatic game over its nuclear ambitions. Rather than giving a flat "No" to the six-nation package of incentives that would be on offer if it gave up its nuclear enrichment, Iran has come back with a more nuanced reply - the top line of which appears to be an offer of "serious talks". This struck a rather different note from the defiance expressed by Iran's supreme leader, Ayatollah Ali Khamenei, only the day before and left the recipients scrambling to co-ordinate their response.

Yesterday every one of the six nations to which the letter was addressed - the US, the EU "troika", Russia and China - referred to the need to give Tehran's response careful consideration. Even the United States, which has often seemed in such a hurry to force the issue back to the UN Security Council, seemed intent on taking its time. The White House said that Iran's response was being given the benefit of a "careful review, as it deserves". The spokesman also went out of her way to say that the US was working with its allies.

If this emphasis on reflection is not necessarily a sign that the US has abandoned its apparent preference for coercion, it is a rather better initial reaction than the knee-jerk negativism that might have been expected. If the US really is concerned to remain in step with its partners, this is also a shift to be welcomed. Any progress in talks with Iran is unlikely to make headway unless the six countries that have offered the incentives package can maintain a united front.

The six - but especially the United States - also need to acknowledge more openly than they have that the quarrel with Iran is, above all, a trust issue. When Iran insists, as it has hitherto, that, as a signatory to the Nuclear Non-Proliferation Treaty, it has a right to develop a nuclear capacity for peaceful purposes, it is a point that cannot be contested. There cannot be one set of treaty requirements for Western countries, or those too big to challenge, and a separate set of requirements for the rest. This would be an invitation to Iran and others to renounce the treaty and pursue their nuclear ambitions without international oversight.

If we believed that Iran's more diplomatic tone, and the greater restraint evident in Washington reflected a genuinely new willingness on both sides to reach agreement through negotiation, we would be more optimistic than we are. In fact, we fear that the caution on the US side reflects, in part at least, a recognition that the international balance of influence has changed. This is a change, it must be said, that does not necessarily bode well for the prospects of peace in the region as a whole.

As a new Chatham House report concludes, the war in Iraq has demonstrated the limits of US power, and Iran has been the chief beneficiary. Israel's recent failure to disarm Hizbollah by force and Syria's refusal to have international peacekeepers on its borders are proof of how Iran's regional influence has grown.

One of the report's key findings is that Iran's influence in the Middle East has now eclipsed that of the US. This is not at all what the US and its British ally intended when they set out to topple Saddam Hussein. By over-reaching and then mishandling Iraq so disastrously, the United States has severely limited its options for constraining a resurgent Iran.