NATO leaders gather in Vilnius this week aiming to overcome divisions on Ukraine's push for a path to membership and end Turkey's block on Sweden joining the transatlantic military alliance.
With the war in Ukraine still casting a dark shadow over Europe, the summit in the Lithuanian capital on Tuesday and Wednesday will be guarded by Patriot missile batteries from Germany, fighter jets and forces from 17 nations.
U.S. President Joe Biden, Turkish President Tayyip Erdogan, German Chancellor Olaf Scholz, French President Emmanuel Macron and British Prime Minister Rishi Sunak will be among the 31 NATO leaders attending the summit in the small Baltic state.
Ukrainian President Volodymyr Zelenskiy is also expected to attend and to press for Ukraine to be admitted into NATO soon after the war triggered by Russia's invasion comes to an end.
"We are talking about a clear signal, some concrete things in the direction of an invitation," Zelenskiy said on a visit to Prague last Thursday. "We need this motivation."
NATO members in Eastern Europe have voiced strong support for Ukraine's stance, arguing that bringing Kyiv under the alliance's collective security umbrella is the best way to prevent another war by deterring Russia from attacking again.
But others, such as the United States and Germany, have been far more cautious, wary of any move that they fear could draw NATO into a direct conflict with Russia, potentially sparking a global war.
"I expect our leaders to reaffirm that Ukraine will become a member of NATO and unite on how to bring Ukraine closer to its goal," NATO Secretary General Jens Stoltenberg, whose term was extended by a year ahead of the summit, said on Friday.
NATO is expected to present a package of support to Ukraine at the summit, including an upgraded cooperation body, the NATO-Ukraine Council, and a bundle of non-lethal military aid to help Kyiv's armed forces to reform and reach NATO standards.
But Zelenskiy has insisted Ukraine also wants assurances on membership that go beyond a vague pledge, made in Bucharest in 2008, that Ukraine would become a member of the alliance.
Negotiators worked through the weekend to try to find language acceptable to all NATO members for the summit declaration but had not finalised the text by Monday, according to diplomats.
"I'm confident that we'll find a united way ... to address the specific issue on membership," Stoltenberg said.
One option under discussion is to declare Ukraine could skip a procedure used by many countries to become NATO members, known as a Membership Action Plan (MAP).
Some major NATO nations, such as the United States, Germany, France and Britain have been holding parallel talks on providing long-term security commitments to Ukraine, focusing on pledges to keep supplying weapons and ammunition.
It was not clear whether a declaration intended to serve as a foundation for these assurances would be agreed before the summit, according to officials.
TURKEY, SWEDEN TALKS
Russia's invasion of Ukraine in February last year has revitalised NATO, the North Atlantic Treaty Organization, founded in 1949 to deter and defend against the Soviet Union.
The invasion prompted Finland and Sweden to abandon decades of military non-alignment and apply to join NATO. In Vilnius, Finland will attend its first NATO summit as a member. But Sweden's accession has been held up by Turkey.
Turkey accuses Sweden of not doing enough to crack down on Kurdish militants but Stockholm says it has met its commitments under an agreement with Ankara - a view endorsed by Stoltenberg.
Erdogan said on Friday in a thinly veiled reference to Sweden that Turkey would not trust a country that harbours "terrorists", adding he would adopt "whatever decision is favourable" for Ankara at the summit.
On the eve of the summit, Stoltenberg will host Erdogan and Swedish Prime Minister Ulf Kristersson for talks to try to overcome their differences.
At the summit, the NATO leaders are also expected to agree they should all spend at least 2% of national GDP on defence - an upgrade on a 2014 pledge to move towards that number.
Currently, only 11 of NATO's 31 members meet the target.
The leaders are also expected to sign off on NATO's first comprehensive military plans since the end of the Cold War to defend against any attack from Russia, setting out detailed tasks and requirements for forces across the alliance.
Turkey has also been holding up adoption of the plans over names used for some geographical locations - reflecting long-running disputes with Greece and Cyprus - but officials say they are confident the plans will be endorsed at Vilnius.
The NATO leaders will also meet with counterparts from Japan, South Korea, Australia and New Zealand as Washington presses the alliance to play a greater role in countering China.
(Additional reporting by John Irish, Andrius Sytas, and Huseyin Hayatsever and Burcu Karakas in Istanbul; Editing by William Maclean)