By Jonathan Marcus
BBC diplomatic correspondent
This Nato summit looks set to overturn earlier expectations.
Just a few short weeks ago the view was that the Bucharest gathering would essentially be about Afghanistan, with a small amount of ceremonial on the side as three additional countries, Albania, Croatia and Macedonia, were asked to begin the formal process of Nato membership.
And, just to prove that the Alliance had not
exhausted its appetite for expansion, Ukraine and Georgia - countries that were once integral parts of the Soviet Union - would be granted their own Membership Action Plans - a programme of enhanced bilateral ties with Nato that would put them in the waiting room for eventual Alliance membership.
But now after a turbulent fortnight of diplomatic exchanges, the Nato enlargement agenda looks far from certain.
True, the US President George Bush has loudly proclaimed during a stopover in Kiev that Nato's door remains open to countries like Ukraine.
But the volume of the US rhetoric should be read as something of a consolation prize to Ukraine and Georgia.
Closer ties with Kiev and Tbilisi proved too much for several European Alliance members, including the French.
The Germans, in particular, put their foot down.
Yes, Nato's door remains open but Berlin clearly believes that this is not the moment to antagonise Russia.
The French wonder at just how far east Nato should go.
And several allies question whether Ukraine, where the population is - to say the least - deeply divided about Nato ties, or Georgia, which still has outstanding territorial disputes involving Russian-backed enclaves, yet meets the criteria for closer relationship.
What remains in the balance is just what kind of new deal, if any, might be offered to these two favoured partners.
On the membership front the picture is really no clearer.
A long-standing dispute between Greece and Macedonia over the latter country's name, which is also the name of a province of Greece, remains unresolved as the summit approaches.
This could mean that only two countries - Croatia and Albania - are invited to join.
If Greece relents, perhaps it might waive its veto of an invitation to Macedonia, on condition that the name issue is resolved before these three new members actually join Nato probably at the next summit in a year's time.
But intense US diplomacy has so far failed to find common ground.
Nato allies have been eager to see Macedonia on board as yet another element in the effort to stabilise the Balkans. Kosovo - not on the original Nato agenda - may also be touched on at his summit.
Kosovo's independence continues to present thorny issues
There is a realisation in Nato that things haven't turned out quite as planned in the wake of Kosovo's declaration of independence from Serbia.
There is a fear in some quarters of a slow drift towards partition - this is certainly what the Russians seem to want - with the northern Serbian enclave perhaps splitting away to join Serbia-proper.
This kind of territorial realignment is precisely what the British and the Americans want to avoid, fearing the precedent it might set.
Kabul on agenda
Afghanistan will be touched on at length, with all of the countries contributing troops or significant financial resources present.
So too will be President Karzai of Afghanistan and the UN Secretary General Ban Ki- Moon.
The goal will be to better harmonise military and civil reconstruction efforts and to try to get the UN to do more of the heavy lifting.
An additional troop contribution is expected from France.
Though welcome, this will only serve to highlight once again the discrepancy in burden-sharing between a small group of Nato nations whose forces are actively engaged in combat operations and the rest.
Mood in Moscow
The Russian President Vladimir Putin is also expected to attend the summit for a session of the Russia-Nato Council; his first and now valedictory visit to such a gathering.
With Mr Putin due to speak, Russia's presence is set to overshadow events
His desire to make some public remarks at the opening of the session has worried certain allies.
But the mood music coming from Moscow is that his comments will be measured; he is after all due to go on for a bilateral summit with President Bush after the Bucharest meeting ends.
All the indications are that Moscow and Washington may be ready to agree a new strategic framework for their relationship, which will inevitably set the context for contacts between their two successors.
It is amazing how far a resurgent Russia overshadows this summit.
Questions of Nato enlargement, its relations with Georgia and Ukraine, Kosovo, and attitudes to US missile defence plans, all have a Russian dimension.
(Nato is expected to give formal backing to the missile defence plans in principle).
Critics argue that certain Alliance members, mindful of Russia's grip on energy supplies to western Europe, are caving in to pressure from Moscow.
It is hard to see a new wave of appeasement towards Russia.
But there is within many of the countries of what former US Defence Secretary Donald Rumsfeld called "old Europe" - the established democracies of western Europe - a new mood of caution.
People don't want to cave in to the Russians but they don't want unnecessarily to antagonise them either.
There's no getting away from it, Russia is back in the European security debate.
And just to prove it President Putin is going to be here.