BRUSSELS — NATO must be ready to intervene externally, experts said Monday, proposing a new doctrine for the alliance, as well as ensure the security of its 28 member states internally, as it has done since 1949.

“In the coming decade, NATO will have four central inter-related military missions,” the experts’ group said in its ‘New Strategic Concept’ for the North Atlantic Treaty Organisation.

Among these is to “deter, prevent and defend” against aggression so as to ensure the political independence and territorial integrity of all 28 NATO member states, conforming to the alliance’s original 60-year-old mission.

The experts also call for cooperation with partners to protect the treaty area against unconventional security challenges.

But the report also insists on the need to send out military missions beyond the treaty area “when required to prevent an attack on the treaty area or to protect the legal rights and other vital interests of Alliance members” — the sort of expedition already seen in Afghanistan, a mission provoked by the extraordinary September 11 attacks in the United States in 2001.

This point was addressed in the previous strategic plan in 1999, noted former US secretary of state Madeleine Albright, who chaired the latest expert group.

Finally, a more global goal is to “help to shape a more stable and peaceful international security environment” by providing military and police training, coordinating military assistance, and cooperating with the governments of key countries.

The text was submitted to ambassadors of the NATO nations for consideration.

The new concept will then be examined and amended before being submitted for adoption by allied leaders at a November summit in Lisbon.