NATO leaders meeting in the Lithuanian capital have much to discuss, as unprecedented challenges beset the US-led military alliance.
Lithuania will host a NATO summit on Tuesday and Wednesday that is set to be dominated by the alliance's response to Russia's invasion of Ukraine and Kyiv's membership application.
These are the main issues leaders will discuss during the two-day summit in Vilnius.
Ukraine's NATO accession
President Volodymyr Zelenskyy is expected to come to the Lithuanian capital with a message for NATO's 31 leaders: Ukraine should join NATO once the war is over.
Kyiv, along with its Eastern Europe allies, is calling for a clear roadmap, arguing it is crucial for Ukraine to join NATO's protective umbrella to deter Moscow from launching future attacks.
But Washington and Berlin are reluctant to go much further than a promise made by the alliance that Ukraine would join one day, without specifying a timetable.
Diplomats have been trying for several weeks to find a formula for the final message that would send a positive message to Ukraine.
Leaders are set to strengthen political ties by launching a NATO-Ukraine Council and setting up a multi-year program to help Kyiv come closer to Western military standards.
But there are questions about whether this will be enough to satisfy Zelenskyy.
"Zelensky's team will push until the last moment to get as much as possible," said Orysia Lutsevych of the Chatham House think tank.
As a prelude to membership, several heavyweights of the alliance - the United States, United Kingdom, Germany and France - are mulling possible long-term arms supply commitments to help Ukraine to defend itself.
Such commitments would be made outside of NATO's framework, according to diplomatic sources.
The weapons promises would complement tens of billions of dollars of equipment already delivered to Ukraine since Russia invaded just over 500 days ago.
An agreement similar to the one between the US and Israel - in which Washington pays several billion dollars a year in military aid to the Jewish state - is among the possibilities mentioned.
Agreements for intelligence sharing, training and reconstruction of the Ukrainian arms industry could also be announced.
But some countries have warned these commitments cannot be a substitute for Ukraine's NATO accession or help delay it.
"The best guarantee of security for Ukraine is full membership in NATO once the war is over," said Latvian Prime Minister Krisjanis Karins recently.
Sweden's accession - at last?
The unpredictable Turkish President Recep Tayyip Erdogan will once again be in the spotlight at the NATO summit.
Allies are insisting he drops his objections to Sweden's membership, which wants to become the 32nd member of the alliance.
Though Ankara gave the green light to Finland's membership in April, it is still blocking that of its Nordic neighbour. All NATO members must agree before a new one can be admitted into the club.
A year ago, at the previous NATO summit in Madrid, it took hours of negotiation to wrest support from Erdogan for the initial invitation to Stockholm.
A meeting between the Turkish President, Swedish Prime Minister Ulf Kristersson and NATO Secretary General Jens Stoltenberg is scheduled for Monday in Vilnius.
They will unblock the situation and obtain a Turkish promise just before the opening of the summit.
After the annexation of Crimea by Russia in 2014, NATO member countries pledged to devote 2% of their GDP to military spending by 2024.
Eleven of the 31 member countries are expected to meet or exceed this threshold this year, according to the latest figures from the Alliance.
Jens Stoltenberg said he was confident that this figure would "increase significantly next year".
Allies have since renegotiated this commitment upwards. The 2% threshold will now be a minimum.
Yet many questions remain undecided, such as the timetable, with some countries estimating it will take them many years to get there.
The Russian invasion of Ukraine prompted NATO to thoroughly review its defence systems on its eastern front.
Leaders have drawn up regional plans adapted to the new geostrategic context, which detail the main threats, the necessary means to defend each region and possible courses of action.
Diplomats say Turkey has objected but should eventually give the go-ahead at the summit.