The leaders of the 28 NATO countries will gather for the first time in two years starting Thursday in Wales, in western Britain, with an agenda very different from the one they had for their last summit.
When NATO leaders last met in Chicago, Afghanistan was at the top of their agenda, and Russia was considered a partner in addressing issues like terrorism and nuclear weapons proliferation. This year, responding to Russia's annexation of Crimea and its invasion of eastern Ukraine will top the agenda.
The alliance faces a "changed world," according to NATO Secretary-General Anders Fogh Rasmussen.
"We are faced with the reality that Russia considers us an adversary, and we will adapt to that situation," said Rasmussen.
That means a greater NATO military presence in member countries close to Russia, like troops in Lithuania, and a new force designed to strike hard and fast in response to an attack on any member.
But NATO has also been clear that it won't go to war with Russia to protect non-members, like Ukraine - drawing a “red line” that Xenia Wickett of London's Chatham House says may be counter-productive.
"One of the things that's going to be interesting not just at the summit but more broadly is whether actually NATO tries to be a little bit more ambiguous with its red lines in the future, which actually gives it a little bit more freedom to act, but also makes potential adversaries a little bit more nervous about acting because they don't quite know when they're going to elicit a strong response," said Wickett.
NATO leaders will also discuss countering the threat from the Islamic State group that has taken over parts of Iraq and Syria and terrorized local people and foreigners. The United States has been bombing the group. And the U.S. and other NATO members have been providing humanitarian aid.
Afghanistan will still be a topic, as Taliban forces are becoming more assertive. NATO's combat mission there will end in December, and officials need Afghanistan's final approval for an agreement on a future advisory and training force.
Even just a few months ago, there was frequent questioning of whether NATO is still relevant in the post-Cold War world. Such talk has been rightly silenced, says Chatham House director Robin Niblett, who chairs an alliance advisory group.
"This is a summit in which all NATO members and their partners need to step up and start to communicate to themselves and externally and to their publics. This is a dangerous world, and there is a purpose to this alliance," said Niblett.
Experts and NATO leaders hope that the severity of the conflict over Ukraine has convinced people in alliance countries to put more money toward defense, and to fund rapid response capabilities designed to confront new threats to Europe and North America.