US President Donald Trump and his Russian counterpart Vladimir Putin are holding a summit in the Finnish capital, Helsinki. Why is this so hotly anticipated?
The US and Russia have long been adversaries but accusations that Moscow interfered in the US presidential election in 2016 have added an extra, bitter ingredient.
Let's take a look.
Why are there US-Russia tensions?
It goes back to the so-called Cold War (from 1945 to 1989) and the hostilities between the US and the then Soviet Union.
They never fought each other directly but differences remained even after the collapse of the Soviet Union and the consolidation of the US as the world's sole superpower.
Fast forward to now, and Mr Putin has made no secret of his determination to reassert Russian power after years of perceived humiliation, often putting his country on a collision course with the US.
Difficult at the best of times, bilateral relations have deteriorated significantly since Russia annexed Crimea from Ukraine in 2014. That led the US and others to impose a series of economic sanctions on Russia.
Why is a meeting between these men so important?
Their relationship has become one of the most scrutinised in global affairs, because of claims of Russian meddling in the 2016 US election, which has been denied by Moscow.
US intelligence agencies believe Russia tried to sway the election in Mr Trump's favour.
An investigation into what Russia did and whether any of the Trump team helped them is the subject of an investigation by Special Counsel Robert Mueller, dismissed by the president as a "witch hunt".
Repeatedly he has called it a Democratic conspiracy driven by bitterness at losing the election.
Since taking office in January 2017, President Trump has sought to improve ties, a stance at odds with traditional Republican party policy.
Last month, he supported Russia's re-admission to the group of industrialised nations - now called G7 - after its suspension following the annexation of Crimea.
What have they said about each other?
Mr Trump has made several comments praising Mr Putin. "Very much of a leader," he said in 2016, "far more than our president has been a leader," in reference to Barack Obama.
Last year, he called Mr Putin a "tough cookie".
In March, he congratulated Mr Putin for his controversial election victory, despite warnings from his advisers not to do so.
Mr Putin has been more guarded in his views about Mr Trump, but has called him a "very bright person, talented" and a "colourful" man in the past.
What will they discuss?
Official statements have lacked details but the talks are likely to include:
- Arms control: Both leaders have bragged about their nuclear capabilities and experts say this is one of the key points to watch. US and Russia have a deal called New Start, aimed at reducing and limiting the size of their nuclear arsenals, the two largest in the world. It is in effect until 2021 and any progress in extending it will be seen as a good sign. They are also likely to discuss a missiles treaty signed in 1987 amid mutual accusations of breaches
- US sanctions: Those were imposed on companies and individuals over Russia's annexation of Crimea, its support to separatists in eastern Ukraine, its role in the conflict in Syria and its alleged interference in the 2016 election. Congress needs to approve the easing of restrictions but observers say Mr Trump can indicate that the list of those sanctioned will not be expanded, a move that would be welcomed by Russia
- Ukraine: The US has given military aid to Ukraine and Mr Putin would be happy to see it scrapped. This, as well as a recognition of Russia's annexation of Crimea, is unlikely to happen. But both leaders can agree to allow international peacekeepers to patrol eastern Ukraine, where a conflict has killed more than 10,000 people
- Syria: Israel, a key US ally, wants to see Iran and Iranian-backed forces away from south-west Syria, in the area next to its border. Mr Trump is likely to raise the issue but analysts say it is not clear whether Mr Putin can make any offer that includes limiting Iran's activities in the country, for example
Why are Trump's allies worried?
During a summit with Nato countries last week, Mr Trump signed a joint statement condemning "Russian aggression".
The question many now ask is whether he will raise the concerns of the allies directly with the Russian president.
It has been widely reported that the European partners have not been briefed about what Mr Trump is really trying to achieve in Helsinki.
There is a fear that after a tumultuous trip to Europe he will have some warm words for Mr Putin.
Mr Trump bashed Nato allies over their defence spending, said Germany was "controlled by Russia" because of its gas imports and criticised UK Prime Minister Theresa May's Brexit plan.
What to expect?
It's hard to say. Mr Trump's unorthodox approach to such talks makes any prediction look more like a guessing game, but US advisers have downplayed any chance of major announcements.
Adding an air of mystery is the fact that both will speak in private during their meeting, with only their interpreters expected to be present.
As a gesture of a restart in relations, they could agree to restore their countries' diplomatic presence after tit-for-tat expulsions in the last few years, most recently over the poisoning of a former Russian spy in England.
What does it mean for the rest of the world?
A lot. US and Russia have been in different, or even opposing sides on many critical issues - Syria, Ukraine, Crimea, to name a few - that have a global impact.
Add to that the Western sanctions in Russia that Mr Putin says are "harmful for everyone".
But European countries, perhaps more than others, will be watching it very closely. They are in an uncomfortable situation as they fear the Russian threat, but are to some degree dependent on Russian energy supplies.
Mr Trump singled out Germany over the controversial Nord Stream 2 project that will boost Russian gas deliveries to Central and Western Europe across the Baltic Sea. The route bypasses not only Ukraine but also the Baltic states and Poland - all of those countries oppose it.
This all leaves little doubt that the world will be watching to see what happens on Monday.