A major Nato ceremony takes place in Brussels today to mark the entry of seven former communist countries into the alliance.
The arrival of Bulgaria, Estonia, Lithuania, Latvia, Romania, Slovakia and Slovenia is the biggest expansion in Nato history, taking membership to 26 countries.
Nato was set up in 1949 to defend western Europe against the Soviet Union. Now three Baltic states that used to be part of the Soviet Union are among the newcomers, as Nato's new borders nudge against Russia.
Russia's new foreign minister, Mr Sergei Lavrov, will be among the dignitaries at Nato headquarters in Brussels for the ceremony - even though Moscow has expressed irritation and doubts about the way Nato has evolved.
The inclusion of the three Baltic states of Estonia, Lithuania and Latvia has particularly alarmed the Russians.
The concern is over possible threats of Russia's own security and about the ability of the Americans - the Nato "parent" partner - to operate freely so close to Russia's borders.
Already, since Monday, Nato planes have been deployed to Lithuania, stationed there to act as part of the normal air defence cover for all alliance partners.
Nato has publicly assured the Russians that the move is routine, but defence minister Mr Sergei Ivanov has called it an "aggressive strategy". He has warned he may order a build-up of Russia's defences on its eastern flank.
The United States has reassured Russia there is no intention of deploying "substantial" forces in any new Nato member country.
Today's occasion will be used to reinforce reassurances and bolster if possible alliance co-operation with Moscow, particularly over counter-terrorism and peacekeeping pledges signed in 2002.