A birthday celebration for NATO this week is set to be overshadowed by rows about Turkey's offensive in Syria, France's declaration the alliance is brain dead and uncertainty over US commitment to trans-Atlantic defence.
President Donald Trump and 28 other heads of state and government will travel to the UK to mark the bloc's 70th anniversary alongside Prime Minister Boris Johnson.
The leader of North Macedonia is included because his country will soon become the 30th member state.
Officials want the focus to be on celebrating what they say is the most successful alliance in history; showing NATO's readiness to keep its members safe - including by naming space as a new operational domain; and setting out plans to tackle future threats.
But splits between allies over challenges such as Russia, China and funding mean there is a risk the birthday summit highlights points of division rather than cements a sense of unity.
That said, Baiba Braze, Latvia's ambassador to the UK, is not overly worried.
"As in the family we will always have discussions and disagreements and there will be types of different views because the countries are different," she told Sky News.
"But as within the families we will sort it out and there will always be a solution for any kind of challenge."
Making the atmosphere for the birthday celebrations on Tuesday and Wednesday even more unpredictable, the leaders will be assembling in London in the midst of a general election campaign.
It is not clear whether this will be a blessing or a curse for Mr Johnson, given his close relationship with Mr Trump - a controversial figure in the UK.
The prime minister was asked on BBC's Andrew Marr Show whether he was worried the US leader might say something embarrassing.
Mr Johnson did not answer that question but said: "I am very pleased that we have very great relations with the United States and unlike the Labour Party, who would abandon NATO, we want to strengthen our defences."
Jeremy Corbyn, the Labour leader, has in the past called for NATO to be disbanded, questioning the need for an alliance created to defend against the Soviet Union. It is the party's policy to remain in NATO.
But Mr Corbyn on Sunday backed a call by France's President Emmanuel Macron for a review of the alliance's focus - and a mending of relations with Moscow.
"In NATO we will work for the alliance to reduce tensions in Europe and beyond," he said at a campaign speech.
"That should be the focus of this week's summit in London.
"President Macron is right to press the case for change in the direction of NATO, including the need to de-escalate the conflict with Russia and the wider perspective on the most serious issues and threats to our common security."
His words echo comments by Mr Macron, who used a magazine interview earlier this month to say NATO was "experiencing brain death" .
He wants the alliance to appoint an independent panel of experts to consider its future focus, signalling that he saw terrorism as the common enemy of allies, not Russia or China.
The threat posed by Islamist extremism will be on leaders' minds given the attack on London Bridge on Friday.
However, many member states have no desire for a warming of ties with Russia given its annexation of Crimea, alleged involvement in the Salisbury spy poisoning and alleged meddling in election campaigns in Europe and the United States.
As for Washington, it views the main foreign policy challenge as China.
Mr Trump is expected to use the summit to discuss what he sees as the threats posed by Beijing, including its dominance of 5G telecommunications networks.
A declaration that will be published by NATO at the end of the summit is set to include a mention of China for the first time - not as a threat but as a country that the alliance needs to think about and develop policy on in a nod to US concerns.
With Britain hosting the birthday gathering, NATO is literally coming home.
The alliance's first headquarters in 1949 was a modest-sized building on Belgrave Place in central London, which now houses the Ghanaian High Commission.
The first secretary general of NATO was also British, Lord Ismay.
Back then, 12 founding allies, which included the UK, France, the United States and Canada, agreed to create an organisation based of the principle of collective defence.
It is enshrined in article 5 of the Washington Treaty - an attack on one is an attack on all.
That founding pledge has come under strain since Mr Trump took office almost three years ago, angry at how allies are overly dependent on the US for their security.
At the last major gathering in July 2018, Mr Trump even threatened to pull the US out of NATO unless member states, in particular Germany, started spending at least the minimum of 2% of national income on defence.
Nine out of 29 allies meet that target, with Bulgaria becoming the latest country to achieve it. They all pledged in 2014 to move towards the 2% goal over the next 10 years.
Germany - Europe's largest economy - has said it should hit that mark by around 2031.
Perhaps mindful of the importance of money to Mr Trump, Jens Stoltenberg, the NATO chief, has revealed that European and Canadian defence spending would grow by more than $130bn between 2016 and 2020, up from a projected figure of "more than $100bn".
In other announcements that appear designed to please his US ally, Mr Stoltenberg has also unveiled a $1bn contract for an upgrade of an ageing NATO fleet of US-made surveillance planes.
And he said that Washington will in future pay less into the alliance's common budget for running its headquarters and other operations.
Keeping Mr Trump and Mr Macron happy is not the only challenge for the summit.
Another flashpoint looks set to be a spat that has pitted Paris against Ankara, in particular over Turkey's decision last month to attack Kurdish militias in northern Syria that are partnered with fellow NATO allies in the fight against Islamic State.
Turkish President Recep Tayyip Erdogan and Mr Macron have been trading public insults in recent days. The men, along with Mr Johnson and Germany's Angela Merkel are due to meet to talk about Syria on Tuesday before the summit starts.
The official anniversary proceedings will begin with a reception at Buckingham Palace, hosted by the Queen on Tuesday afternoon.
That will be followed by a second reception at 10 Downing Street, led the prime minister.
The formal working meeting is on Wednesday morning at a hotel resort in Watford, northwest of London. It will only last three hours. Leaders will then speak to the media in an opportunity to show camaraderie or animosity.