A new document for peace, security and shared values in a changing world is how NATO leaders are describing their new strategic concept.
The mission statement also recommits the alliance to defending any of its members that comes under attack. It is just one of the reasons why Nato leaders have agreed to build a missile defence shield.
US President Barack Obama said: “For the first time, we have agreed to develop missile defence capabilities that are strong enough to cover all Nato European territory and populations, as well as the United States.”
The leaders will also invite Moscow to join the system when they meet Russian President Dmitry Medvedev in Lisbon on Saturday.