Despite days, indeed weeks, of speculation, few people in Washington can accurately predict what will happen as US President George W Bush and his Russian counterpart Vladimir Putin begin three days of talks.

We know they will emerge smiling and shaking hands. Mr Putin will, no doubt, be effusive in his praise for Mr Bush's Texas ranch.

But in this usually well prepared world of diplomacy, much of the substance will depend on their personal talks.

President Bush's national security adviser, Condoleezza Rice, has already told us about the changing nature of the relationship. It is no longer, she insists, just about missiles and mutually assured destruction.

Yet to fulfil her prophecy, the two leaders need to reach one last major agreement on just that topic.

The outlines of a grand compromize over missile defence have already emerged.

On missile defence itself, Russia would allow American tests to go ahead in return for keeping the 1972 Anti-Ballistic Missile treaty in place - at least formally.

On the parallel, but linked, issue of missile cuts, the two sides would agree in principle to massive cuts in their arsenals - bringing them down to between 1,750 and 2,250 warheads.

US concessions

Whatever Washington may say, it is the Americans who will be doing most of the compromizing.

Russia's missile arsenal is likely to go down to below 1,000 warheads anyway. And on missile defence, the diehards in the Bush administration would dearly love to see the end of the ABM treaty altogether.

If Washington swallows the compromises, the reason, for sure, will be the desire for better relations with Moscow, especially for the war on terrorism.

Yet it is not nearly so easy.

Even the most imaginative lawyers are having trouble reinterpreting the ABM treaty to allow tests which are clearly forbidden.

As Senator John Kyl, a leading supporter of missile defence, put it:

"If you call a tail a leg, how many legs does a dog have? The answer is still four.

"Saying the ABM treaty permits testing doesn't make that true. It doesn't, and there's no provision within the treaty for waiving its terms or consenting to allow a violation of the treaty."

And forget about amending the treaty - the US Senate would never let that through.

Vague words

As for missile cuts, the issue is simply too complicated to be negotiated away in a matter of weeks.

The Russians, for example, want to cut both the numbers of warheads and the number of "delivery vehicles". The Americans want to keep their delivery vehicles and convert them for conventional use.

And the missile defence issue is just one of seven separate agreements the two leaders are hoping to reach.

Some commentators in Washington are seeing this as one of the most important summits in recent years, as Russia and the United States attempt to chart a new course for the post post-Cold War world.

But expect some very vague wording, as the two sides attempt to finesse their many remaining differences with some broad expressions of intent.