Nato's mission is evolving as more countries join
And then there were 26.
This is the second great round of Nato expansion.
Since the end of the Cold War, Nato's European frontiers have moved steadily eastwards; first taking in Poland, Hungary and the Czech Republic, now extending all the way to Romania and Bulgaria's Black Sea coast.
All of this has taken place with relatively little drama and with great speed.
The Baltic republics used to be part of the Soviet Union. Now they are to be fully-fledged members of Nato.
In less than 15 years since the Berlin Wall came down, the strategic map of Europe has been transformed.
Benefits and obligations
The new members gain security as part of a collective defence organisation - although the threat of invasion is now remote.
The three Baltic republics - Estonia, Lithuania and Latvia - have fairly small armed services and Nato has urged them to put their limited resources into deployable forces which could contribute, for example, to a peacekeeping battalion.
The Pentagon is also keeping a close eye on Nato enlargement; it sees all sorts of advantages in moving its military 'footprint' in Europe eastwards
In due course, the policing of the air space of the Baltic republics will become a Nato responsibility.
Belgium is to provide four F-16 jets and their pilots, who will operate from a base in Lithuania.
Norway and Denmark will provide ground handling and air-traffic control facilities for the operation.
But Nato is not just about benefits, it is also about obligations.
Several of the new members have a strong record of contributing to international peacekeeping operations. Both Romania and Bulgaria have significant contingents - for example in Iraq.
For many, alliance membership is also a rite of passage; a confirmation of their own transformation into democratic, market-orientated states.
Nato says that its door remains open to more new members; the prime ministers of Albania, Croatia and Macedonia are attending Monday's ceremonies in Washington.
One day perhaps, most of the Balkans could be within the Nato fold.
But Nato, too, is changing.
It is taking on new missions in Afghanistan and maybe even eventually in Iraq.
It is increasingly looking towards its southern flank with North Africa, amidst growing concerns about terrorism.
Nato warships are already involved in counter-terrorism patrols in the Mediterranean.
And the whole issue of the alliance's outreach towards the greater Middle East will be one of the main items on the agenda of the Nato summit, scheduled for Istanbul in June.
The Pentagon is also keeping a close eye on Nato enlargement.
It sees all sorts of advantages in moving its military "footprint" in Europe eastwards.
And Washington is already eyeing the territory of some of the new Nato members as potential locations for military bases from which to project US power into the greater Middle East.