Nato is welcoming three Balkan nations into the fold, as part of an effort to consign the region's recent wars and troubles to history.
Bosnia signed on 11 years after the end of its war
Serbia, Bosnia-Hercegovina and Montenegro are entering the alliance's Partnership for Peace programme, putting them on the path to membership.
Ceremonies were held on the 11th anniversary of the Dayton Agreement, which ended the war in Bosnia.
Other Balkan nations will become full members of Nato in coming years.
Slovenia became the first of the former Yugoslav nations to achieve full membership two years ago.
Croatia and the Former Yugoslav Republic of Macedonia are expected to join in 2008.
At a ceremony to induct Bosnia, Nato Secretary-General Jaap de Hoop Scheffer said: "Bosnia-Hercegovina has finally moved from the Dayton era to the Brussels era."
Bosnian Serb leader Nebojsa Radmanovic said more than a decade after the war, "the dream of Bosnia-Hercegovina has come true".