Jonathan Marcus
Diplomatic correspondent
BBC
Joint military exercise of forces from Georgia, Britain and the USA
Nato was set up to counter the threat of post-War Communist expansion
Nato will celebrate the arrival of its latest member, the small Balkan state of Montenegro, as a sure sign of the alliance's continuing relevance at a time of renewed tensions in Europe.
The decision to admit Montenegro must next be ratified in each of the alliance's national capitals - but the basic decision has been taken.
Montenegro's contribution to the alliance's defences will, in fairness, be limited.
It has a tiny population and an active-duty military of only a little more than 2,000 members.
But, as Nato says: "Montenegro has provided troops for the training mission in Afghanistan and financial support to the Afghan security forces."
"Montenegro, also plays a constructive role in the Western Balkans region."
This points to another aspect of Montenegro's perceived value as a Nato member.
After the bitter Balkan wars, Nato is slowly bringing the new democracies of the region into its fold; a means of spreading a security blanket across Europe's once fractious south-eastern flank.
Nato's expansion since the end of the Cold War has been dramatic.
As a journalist covering defence matters during the late 1980s, I used to receive a regular copy of a journal called Nato's Sixteen Nations.
Once the Berlin Wall came down and Nato's doors were opened, it had to change its name with the arrival of each new member, and then it gave up.
Today, with Montenegro, there will be 29 Nato members in all.
Two countries, Macedonia and Bosnia-Herzegovina have been offered what Nato calls membership action plans, a kind of waiting-room for membership.
Nato:
Formed in 1949 to counter the threat of post-War Communist expansion as the Soviet Union sought to extend its influence in Europe
Originally consisting of 12 countries: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the UK and the US
The organisation expanded to include Greece and Turkey in 1952 West Germany in 1955, and Spain in 1982. However, then, as now, the alliance was militarily dominated by the US
The Czech Republic, Hungary and Poland became the first former Warsaw Pact countries to gain Nato membership, in 1999
Currently has 28 members. Before Montenegro's expected admission, the most recent recruits were Albania and Croatia in 2009, and Estonia, Latvia, Lithuania, Slovenia, Slovakia, Bulgaria and Romania, which joined in 2004
Bosnia-Herzegovina's status is still pending until it resolves some issues about ownership of defence property, while the Former Yugoslav Republic of Macedonia - that's what Nato has to call it - is unlikely to join until it resolves Greece's objection to it being called simply Macedonia, which also happens to be the name of a region of Greece itself.
So could Nato membership stall at 29? Nato has grandly stated that Georgia and Ukraine will one day be members.
But a deeply divided Ukraine is not, at present, seeking to sign up, and Georgia has so many problems with Russia (with whom it fought an unsuccessful war not so long ago) that it is almost impossible to see it becoming a member under current circumstances.
Nato may have to look north for new members.
Russia's growing assertiveness is leading to ever closer defence ties between the alliance and Sweden in particular, but also with Finland.
In neither country, though, is there any consensus about actually joining Nato.
Sweden may ultimately feel it can achieve its goals by enhancing its practical cooperation with the alliance.
So, for Nato, these are in some ways the best and the worst of times.
The steady stream of new candidates for membership has reassured the alliance that even with the Cold War over it still has a purpose.
Montenegro's President Filip Vujanovic has been courting Nato membership since coming to power in 2006
Indeed Russia's increasingly aggressive behaviour - not least its seizure of the Crimea and its war in eastern Ukraine - has taken Nato in a sense back to its roots - the defence of the territory of its own members.
After years of counter-insurgency operations in the Balkans and Afghanistan, Nato must again contend with, at least the possibility of, a classic all-arms encounter with Russia.
Defending the Baltic Republics and Poland is now its top priority.
But Russia's war in Ukraine has also revealed significant shortcomings in Nato's military capabilities.
US military experts have watched, for example, Russia's use of electronic warfare on the Ukrainian battlefield with great respect.
Many of the more traditional aspects of high-intensity warfare have been, if not so much forgotten in Western armies, given a much lower order of priority.
There is a good deal of ground to make up.
A billboard with a portrait of Russian President Vladimir Putin is displayed on a street in Kerch,
Crimea, April 016. The board reads: Image copyright
REUTERS
He has questioned Nato's value and has described many of Washington's allies as freeloaders.
Some of Nato's European members are doing more, but many still have a great deal more to contribute.
Its fitting then, perhaps, that the alliance will meet for its next summit in the Polish capital, Warsaw, in early July.
Poland, of course, was one of the key members of the then Soviet-led alliance, the Warsaw Pact.
It is now a stalwart of Nato and eager to see the permanent basing of alliance forces on its soil.
Nato's continued expansion greatly annoys Moscow.
A Nato summit so close to home will annoy the Kremlin even more.
