US Secretary of State Rex Tillerson embarks this week on a diplomatic mission to shore up alliances in Europe and their defences against Russian aggression and subversion.
But as he prepares to set off Monday for Brussels, Vienna and Paris, his authority to speak for Washington has been called into question by a rift with President Donald Trump.
Anonymous leaks from the White House suggest the top diplomat could be replaced within weeks and -- even while denying this -- Trump reminded him: "I call the final shots."
Tillerson has dismissed as "laughable" reports that Trump's closest aides want him to resign, but rumours will inevitably dog him when he sits down with his European peers.
His trip is structured around ministerial conferences of the Nato allies in Brussels and the 57-member Organization for Security and Cooperation in Europe, in Vienna.
But the most closely watched encounter will be a meeting with his Russian counterpart, Foreign Minister Sergei Lavrov, on the sidelines of the OSCE talks late in the week.
At Nato, alliance partners will want to hear how America can help defend them against the newly global threat of North Korea's long-range nuclear missiles.
Tillerson gave a major speech on European policy in Washington on Tuesday to preview his agenda ahead of the trip -- and he did not mince words about Russia.
He will tell his Nato Western allies that they must stand shoulder-to-shoulder to oppose "the enemies that threaten our security and oppose our way of life," while Russia must be brought to reverse its intervention in eastern Ukraine through relentless sanctions pressure.
"Any resolution to the war that does not entail a fully independent, sovereign, territorially-whole Ukraine is unacceptable," Tillerson declared.
"Our trans-Atlantic unity is meant to convey to the Russian government that we will not stand by this flagrant violation of international norms," he said.
But for this warning to be credible, Tillerson must be seen as speaking for the whole US government and the president, who has shown no enthusiasm for sanctions against Russia.
Trump -- who in 2013 suggested he become Russian leader Vladimir Putin's "best friend" -- opposed a law passed this year by Congress that could impose new sanctions from January, over Russia's Ukraine intervention and alleged interference in the US presidential election.
The White House has even attempted to play down the conclusion of US intelligence that Russia deployed hackers and propagandists to assist in Trump's election win.
But since that victory, the Trump administration's other major players -- Tillerson and Defense Secretary Jim Mattis -- have taken a tougher line.
Relations with Russia are at a low after tit-for-tat closures of diplomatic missions, although both sides seem to want to cooperate to stabilize Syria.
Despite the enmity, progress may be made this week in agreeing terms for a United Nations mission in Ukraine.
Putin has suggested Blue Helmet troops deploy to protect the unarmed OSCE monitors struggling to oversee the shaky ceasefire between Kiev's forces and Russian-backed rebels.
Western powers would like to see the force have a broader mandate.
Germany's Foreign Minister Sigmar Gabriel met Tillerson in Washington on Thursday, and afterwards told reporters that they found themselves in agreement on the crisis.
"We believe that we absolutely need a robust mandate by the United Nations for a peacekeeping mission," Gabriel said, warning that tough negotiations lie ahead.
"There are -- not minor, but truly significant -- differences between Russia, the United States and us Europeans," he said.
The UN mission must have the authority to implement the truce and enforce the withdrawal of heavy weapons from the disputed and conflict-torn Donbass region, Gabriel said.
"We share this view with our American partners," the German minister added.
Despite overwhelming evidence of its involvement, Moscow continues to deny that it has played a role in the eastern Ukraine fighting.
On North Korea, Europe had a rude awakening on Wednesday when Kim Jong-Un's North Korean regime demonstrated that his banned missile program can, in theory, strike targets on the continent.
Any high-altitude shield to protect Europe from North Korean missiles flying over the pole could breach non-proliferation deals with Russia, promising a new diplomatic row.
Trump, meanwhile, has already made it clear that he thinks the United States already pays more than its fair share of the costs of Nato's collective defence.
Tillerson is due to arrive in Brussels late on Monday. He has talks with EU and Nato leaders on Tuesday and Wednesday before flying on to Vienna for the OSCE.
Heading home on Friday, he will stop in Paris for talks with senior French officials.