When Joe Biden and Vladimir Putin meet virtually on Tuesday the two presidents will have to negotiate a history of mutual suspicion as they take up the urgent issue of a major Russian military buildup on the Ukraine border.
The key question hanging over the talks -- and the subject of keen debate among analysts and political leaders -- is whether Putin might actually launch a cross-border offensive, or whether he is using the troops to pressure Biden for guarantees ex-Soviet Ukraine will never become a NATO launchpad.
The two have a daunting list of other differences to air, from Russia's harsh treatment of dissidents to the presence of ransomware hackers on Russian soil to Moscow's support for the repressive regime in Syria.
But the magnitude of the Russian buildup near Ukraine -- the Kremlin may be planning an offensive early in 2022 involving up to 175,000 troops, according to US intelligence obtained by the Washington Post and other outlets -- has raised red flags in Washington and across Europe.
Many analysts doubt that Putin would carry through with an invasion -- which would inevitably prompt international condemnation and probably new sanctions -- but at least some take a darker view.
"Putin has sharply raised the stakes. He is no longer bluffing," said Tatiana Stanovaya, founder of the political consultancy R.Politik Center and a nonresident scholar at the Carnegie Moscow Center.
"He's ready to take a desperate step," she told AFP on Sunday.
The looming crisis could pose the sternest test yet of the foreign policy savvy and clout of the 78-year-old US president.
Biden and Putin -- who are expected to speak around midday Tuesday, Washington time -- have history together.
They first met in person in the Kremlin in 2011. Then vice president Biden later said he told the Russian leader, "I don't think you have a soul" (to which, Biden says, Putin responded, "We understand one another.")
They met again in 2014 in Geneva to deal with the now familiar issue of Russian military pressure on Ukraine.
And they met in Geneva on June 16 of this year -- for the first time with Biden as president.
Contacts have continued since -- as have tensions, with Putin seen as eager to pressure Biden into another in-person summit as a way to project parity on the world stage.
On Friday, Biden vowed to make it "very, very difficult" for Russia to launch an invasion, but did not say how.
Putin has warned the West and Kiev against crossing the Kremlin's "red lines," including building up weaponry in Ukraine.
Biden later responded, "I won't accept anybody's red line."
Some analysts said Russia, deeply concerned with Ukraine's warming ties to NATO, is applying pressure to cut that movement short.
Following Putin's lead, Russian Foreign Minister Sergei Lavrov on Thursday called on US Secretary of State Antony Blinken to provide "security guarantees" that NATO would not come closer to Russia's border.
Stanovaya said this might be Putin's bottom line: "Either NATO provides guarantees or Russia invades Ukraine," she said.
Russia has continued to deny any bellicose intentions, instead accusing the West of provocations in the Black Sea.
NATO recognized Kiev in June 2020 as one of a handful of so-called "enhanced opportunity partners," potentially a step toward membership.
Heather Conley, a former assistant US secretary of state for European affairs, said she believes Putin is willing to apply "enormous pressure" in the Ukraine standoff.
He is set on another in-person summit with Biden, said Conley, who is with the Center for Strategic and International Studies. And he wants to loosen Western ties to Ukraine which, she said, some see as "a sort of NATO aircraft carrier."
Fyodor Lukyanov, a prominent political analyst close to the Kremlin, said he doubts Biden and Putin will agree on anything concrete on Tuesday, but he does not expect hostilities to break out if the talks fail.
"No, this is hysteria whipped up by the West," he told AFP on Sunday. "Wars begin suddenly. If it begins, it will begin differently."
Moscow seized Crimea from Ukraine in 2014 and has since backed the separatist forces fighting Kiev. The conflict has left more than 13,000 dead.
What if the virtual meeting between the rival leaders goes poorly on Tuesday?
If Russia fails to obtain the accommodations it seeks, and all efforts at diplomacy fail, said Conley, her sense is that "Mr. Putin would then use military means to achieve his political objective."