President Bush’s televised address last night was redundant: he has put his vanishing credibility on Iraq in the hands of General David Petraeus, and his remarks now have less weight than those of his commander.

Bush says that the 30,000 extra troops who went to Iraq this year for the surge should come home by next summer — provided that conditions permit. But that is to say nothing at all. Petraeus said the same, and added weight by saying that the Army could not stand the strain of a longer deployment, a known constraint from the start of the surge.

Bush will say it again today, from a Marine base at Quantico, Virginia; the White House today will give to Congress (as it must) its status report on Iraq; Bush will use his weekly radio address tomorrow to repeat his message, and the White House is trying to insert officials into the Sunday morning television talk shows. But to say what? To argue for more time in Iraq? Probably — but that is to stretch Petraeus’s presentation too far, beyond the claims that the general himself made.

Looking back, it was an extraordinary account by Petraeus in front of the House of Representatives and then the Senate. The image of the week is his sequence of 13 slides, a perfect example of how data can prove equally useful to opposite arguments.

He pointed out that the violence was down; is that because US forces are getting control, or because warring factions are now segregated? Discoveries of arms caches have risen sharply: good news, because more have been found, or bad, because more arms are in the country? Only about a dozen battalions of Iraqi security forces can operate independently, unchanged in a year — well, that one is only bad news.

His thirteenth slide, the last one, now famous across the US, deserves, like Al Gore’s PowerPoints of global warming, to launch a whole film. It shows US forces in Iraq shrinking steadily, but although the lower axis is marked “Time”, question marks take the place of dates. As they should; Petraeus did not answer the question of why more time will glue Iraq together if it lacks a government committed to that goal.

Petraeus is often credited with political ambitions. You could now construct, as a fourteenth slide, a graph of Petraeus’s career, with Secretary of State or another of Washington’s glittering prizes as the goal, and “Time” along the axis; the main question is how long it will take him.

It may not even depend on who wins the next presidential election. His slides made the case (if not in so many words) that he had at least delivered on his own brief, in using the extra troops for visible impact. But by leaving the causes of that impact open to so many interpretations, his evidence could be deployed to support most policies, other than the abrupt exit of most US forces. It could be used by any leading presidential candidate, on either side.

For that reason, Bush will find it hard to claim that Petraeus’s presentation solidly supports his case. He tried to use Petraeus to do the hard slog of making his case to Congress.

But that is the kind of task best not delegated, as the messenger inevitably appropriates the message. Petraeus’s presentation to Congress made his own case better than Bush’s.

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