Q: Are these the first new members?
It is not the first time the Alliance has accepted new members, but this could be the most crucial enlargement in Nato's history. The new arrivals will bring the number of members from 16 to 19.

Q: Why is this enlargement 'crucial'?
Since the end of the Cold War, Nato has been engaged in a difficult balancing act of trying to modernise itself, cope with the new security concerns of central and eastern European states which have felt in a security limbo since the collapse of the Warsaw Pact, and all of this without alienating Russia.

The advocates of enlargement say the Alliance is spreading stability throughout Europe. The critics complain it's simply drawing new lines of confrontation, and actually creating new instability.

Q: Why were these three picked to join, and other east European countries left out?
The desire of central and eastern European states to be brought under Nato's umbrella has always forged ahead of Nato's willingness to welcome them in:

• First, the Alliance offered dialogue and partnerships.

• In 1994, it agreed to accept new members.

• Just how many became a focus of intense debate within the Alliance. Some European states argued for a large group, but the United States advocated a smaller number with the option of further enlargement.

• Poland, Hungary, and the Czech Republic were eventually chosen as the candidates which best fulfilled the criteria of having established democracies, good relations with their neighbours, and effective armed forces under civilian control.
• The NATO meeting in Missouri will affirm that the door remains open to further enlargement.

Q: Who's next?
There are doubts as to whether there is real appetite in Nato for further enlargement. There are a number of disappointed states, not least Slovenia and Romania, which fear their desire for membership will be delayed indefinitely.

Nato officials accept that they will have to work hard to reassure those who have been left outside that they are not isolated.

But the Baltic states are amongst the keenest on Nato membership, and some argue also those most most in need of security guarantees, and Russia's particular sensitivity in that region make their accession a very distant prospect.

Q: What does Russia think of it all?
Nato officials accept that, despite their best efforts at reassurance, there is still intense mistrust over Nato expansion across a broad sprectrum of opinion in Russia.

Nato has tried to engage Moscow in ever deeper dialogue, while resisting any suggestion that Russia has a veto over its future plans.

But it's clear that it will continue to be a difficult process trying to manage what remain an area of deep disagreement between the two sides.

Q: How much will it cost?
Enlargement means Nato's existing members will be extending security guarantees to their new allies.

In the run-up to enlargement, there has been much argument over how much the process will cost, at a time when all Nato members have been trying to economise on defence, and that the new entrants would be forced to buy new equipment they could ill-afford.

The official US government estimate is that it will cost up to 35bn US dollars in all.

Q: So is enlargement a good thing?
Nato's supporters say it has to adapt or die. They argue that the prospect of enlargement is already spreading stability by encouraging aspiring members to reform their institutions and improve their relations with their neighbours.

Sceptics say that, because of the sensitivity of the issue, Nato is bestowing membership on those countries which need it least, and excluding those which are most vulnerable.