It is a slightly strange but perhaps appropriate arrangement that Remembrance Sunday is followed by the Lord Mayor's Banquet. At one level there is a disconnection between the sombre ceremonies witnessed yesterday and the feast in Guildhall tonight. At another, however, it is right that ceremonies to mark the sacrifice of those who died in conflict precede an event at which the Prime Minister makes an annual address on international relations. The tragic truth is that war is part of foreign policy, even if peace and prosperity are the long-term goal. And that harsh reality must be recognised by Mr Brown.

The attention this evening will probably be on the words and tone that Mr Brown adopts concerning Iran. This is perfectly understandable. The consequences of Tehran acquiring the bomb would be profound. It is right that the Prime Minister has not ruled out any course of action on Iran – including a military response – because a lack of resolve would only encourage extremists there to seek nuclear weapons. If that ambition is not rebuffed, then at some point difficult decisions may have to be taken.

Iran, though, is a comparatively short-term question. Within a few years it will be clear whether that nation has become a nuclear power, or found that the actions of others have made it impossible to pursue an aim that plays to nationalistic ambitions but is inimical to international interest. There is an international flavour to another unpleasant but real threat that the Prime Minister must discuss with candour. He must be blunt about the nature of the struggle with al-Qaeda and its sympathisers and the extent of the British commitment in Afghanistan.

The critical feature of both threats is their timescale. Much progress has been made against al-Qaeda. This is particularly so in Iraq, where those inspired and perhaps directed by Osama bin Laden have this year been badly defeated in provinces where he had support. It would be a great mistake to assume that such defeats will convince religious fanatics that their cause is hopeless and should be abandoned. But there is no doubt that the facts on the ground in Iraq have changed fundamentally in recent months, through a combination of military action and local alliances against the external extremists.

The conflict in Afghanistan is as different as the national circumstances and geography of that country. It would be ludicrous for any politician to say that British troops will be back home by the end of 2008 or 2009. Even if vast progress were made against the Taleban, it would be foolish to conclude that this was an enemy that had been consigned to history. Afghanistan has been a lawless place for a long time and that instability has made it a haven for dangerous terrorists. No matter how bad the plight of its people, any organisation that puts the glamour of martyrdom above the challenges of daily life will hold an unhealthy attraction for the backward or the bent. Britain will be in Afghanistan for many years and at great cost. It has to be. This should be honestly conceded, and the efforts of our troops in harm’s way must be recognised.

Candour is thus the real challenge for the Prime Minister. Foreign commitments of this type cannot be “spun”. It would be better to lay out the rugged reality than pretend that we are living through temporary and minor inconveniences. The public, never mind the Lord Mayor and his guests, are entitled to hear the truth and nothing but the truth.