Leading article: The spectre of Saigon looms over Baghdad

The exact death toll had still to be established last night, but the symbolic significance of the attack was instantly clear. A suicide bomber had successfully penetrated the fortified "green zone" in Baghdad and blown himself up inside the cafeteria of the Iraqi parliament. Three MPs were among the eight or more dead; at least 30 people were injured.

For several weeks now, the US military authorities have argued that an upsurge in violence was only to be expected as the new "surge" tactics started to bite. The enemy, they reasoned, would fight ever more desperately until it was finally overcome. The possibility that there might be a different explanation - that the "surge" might simply not be having the desired effect - was not entertained, at least in public pronouncements.

Yesterday's bombing constitutes a direct challenge to the US strategy in Iraq. The last time bombers successfully penetrated the "green zone" was in October 2004. Since then, the only violence had been at the outermost edges; its formidable security had held. That someone was able to pass through the security checks yesterday with explosives sufficient to inflict so much death and destruction means that the "green zone" can no longer be considered impregnable.

It hardly matters whether, as is suspected, the bomber was a security guard to an MP. The fact is that US forces are responsible for making the zone safe, and its security has now been compromised. All the elaborate fortifications and entry procedures will have to be reviewed.

That it was the Iraqi parliament that was targeted conveys an especially dispiriting message. The Parliament represents the last vestige of US (and British) hopes of planting something even faintly recognisable as democracy. The elections in December 2005 were, in retrospect, the high point of optimism for Iraq's future. Iraqis defied the threat of violence to cast their votes with quite extraordinary heroism. Already, though, their ministers and legislators seem estranged from them. If the Parliament is no longer able to meet, or if so many MPs fear for their security that they stop attending, the last chance for an orderly Iraq governed by Iraqis would seem to be gone for good.

The other risk is that any remaining confidence that the Americans are able to keep their allies safe will be undermined. Inside the "green zone" are not just the Iraqi parliament and many US military and diplomatic facilities. The "zone" is also home to many Iraq government offices and foreign representations; several thousand Iraqis live there. If the "zone" is seen to be vulnerable, all trust in the possibility of order spreading out from there to the rest of Baghdad will evaporate. The spectre of a Saigon-style retreat from Baghdad will be harder and harder to dispel.

This is the ninth week of the US "surge". More and more American and Iraqi soldiers are to be seen on Baghdad streets, as the attempt to crack down on the violence gains pace. The greater visibility of US troops, which is an integral part of the strategy, automatically makes them more vulnerable. It is probably inevitable that, even as the number of violent incidents has declined, US military casualties have increased.

For the strategy to work, it must do much more than multiply armed patrols. It must convince Iraqis that law and order can be restored, not just now but in the longer term. It is not just about deterring gunmen and bombers; it is about instilling confidence in the authorities' prospects of success and reducing support for militant sectarianism. The US "surge" already seemed to be in trouble; yesterday's bombing showed that the citadel could be breached. If and when the US abandons Iraq, this day will mark the beginning of that end.