Foreign ministers of the three new members of the North Atlantic Treaty Organisation - the Czech Republic, Hungary and Poland - will deposit documents ratifying their countries' acceptance of the treaty terms with the US Secretary of State, Madeleine Albright. It will mark their formal accession to the alliance - and their protection under the mutual security clause they so coveted during the 40-year stand-off between East and West.
The key section of the treaty - Article 5 - says "an armed attack against one or more of them in Europe or North America shall be considered an attack against them all".
The State Department says the article comes into force the moment the foreign ministers hand over their countries' instruments of ratification, even though full military integration will take years.
The documents themselves will ultimately be deposited in the National Archive in Washington.
The choice of Independence and the Truman Library as the venue for the formal accession of the first post-Cold War Nato members was the personal wish of Ms Albright. Truman was president when the US presided over the creation of Nato and then masterminded it through a Congress reluctant to endorse military involvement overseas.
His speech at the signing of the original treaty - on 4 April 1949 - and that of his secretary of state, Dean Acheson, are lodged at the Independence library.
The State Department says the three new Nato states appreciate the historical significance of the venue and wanted to join Nato formally as soon as possible after ratifying the treaty and meeting the basic military requirements. Today's ceremony thus meets the interests and aspirations of all the parties.
In recent weeks, however, there have been rumblings of discontent from some of the new member states' Western backers. Their feeling is that a landmark of 20th century history, and one of President Bill Clinton's few foreign-policy triumphs, is being secreted away from public view. They complain that it is being deliberately separated from next month's festivities in Washington, which will both mark the 50th anniversary of Nato and set its future course.
A senior State Department official denied that there was any ulterior motive for separating the two occasions.
The Czechs, Poles and Hungarians, he said, wanted to be able to participate as full members in the planned discussions about Nato's future, which would be easier if they were already full members.
He offered no reason why the ceremony could not have been held, for instance, a day before the main gathering, but still clearly within its framework; nor did he explain why it was not held in the vicinity of the White House, where the foreign ministers of the first 12 members signed the original treaty.
For the suspicious, there is a one-word explanation: Russia. For Russia, the expansion of Nato is a bitter pill.
But Russia, as a Nato "partner", will be one of 44 countries represented in Washington at next month's commemoration of Nato's 50th anniversary, an event billed by US diplomats as "charting the way for a larger, more flexible alliance in the 21st century".
While the intention may be to demonstrate to Russia that its security is not threatened by a larger Nato, for some there is a paradox.
The inclusiveness of Nato's birthday party will have been made possible by today's ceremonial in Independence, which perpetuates the very division that Nato expansion was intended to banish.Reuse content