There was a time when Marco Rubio regarded President Vladimir V. Putin of Russia as a “gangster,” a “war criminal” and a “thug.”
By his own admission, Mr. Rubio, now President Trump’s secretary of state and national security adviser, has since undergone a transformation on foreign policy. Once quick to criticize the wing of the Republican Party skeptical of foreign interventions and those who would seek accommodation with Russia, Mr. Rubio began adapting to Mr. Trump’s views years ago.
He is now helping steer the administration through fitful negotiations with the Kremlin to bring an end to the bloody assault on Ukraine begun by Mr. Putin.
Mr. Trump has now delegated to Mr. Rubio an especially tricky job: negotiating with European leaders to produce proposed security guarantees for Ukraine.
It is perhaps the highest-profile role Mr. Rubio has taken on. It will require him to balance Mr. Trump’s insistence that no American troops be stationed in Ukraine against the need to reassure President Volodymyr Zelensky of Ukraine that his country will have backup should Mr. Putin violate any peace deal and attack again.
Mr. Rubio will have to persuade the Europeans that the mercurial Mr. Trump will stick to his commitment to help them defend Ukraine without doing so under the banner of NATO. And even though Mr. Trump asserted recently that Mr. Putin had accepted Ukraine’s need for security guarantees, Mr. Putin’s foreign minister said on Wednesday that Russia would insist on being part of any security plan for Ukraine, making the process of creating a proposal that could lead to a peace deal that much more fraught.
Mr. Rubio will lead what a senior administration official called “sensitive diplomatic conversations” with counterparts from Ukraine and other European allies. The national security advisers from those countries plan to meet on Thursday with Mr. Rubio, according to one person familiar with the plans.
An administration official said the talks would determine “what the security guarantees could look like” before a potential meeting between Mr. Zelensky and Mr. Putin.
A security guarantee could encompass a wide range of issues. In return for Russia ending its invasion, a security pact could include a pledge of U.S. air support for any European-led operations should Russian troops resume their assault.
It could comprise a pledge of U.S. assistance in intelligence, surveillance and reconnaissance to support missions in Ukraine and a naval force to keep the Black Sea, including access to the Danube, from falling under Russia’s domain. And it could address how to ensure that any security guarantees would be legally binding, including whether they would need congressional approval.
“From a European perspective, I think everyone is quite happy that Rubio is leading the working group, because he’s definitely perceived as the one who is most competent to come up with something that could actually work,” said Liana Fix, a fellow for Europe at the Council on Foreign Relations.
She said that European leaders hold a lower view of some other Trump administration officials, but Mr. Rubio has forged positive relationships among the Europeans for years through his time in the Senate, where he was a member of the intelligence and foreign relations committees.
“There will be no troops on the ground, but what Europeans have always asked for is logistical support, especially when it comes to intelligence, because Europeans basically don’t have that capability themselves,” she said.
The move to entrust Mr. Rubio with the sensitive task highlights his gradual transformation from a hard-line Russia hawk to a Republican who views the world more through Mr. Trump’s “America First” lens. But it is also a testament to Mr. Rubio’s rising stock within the administration.
Mr. Rubio has been privy to Mr. Trump’s calls with Mr. Putin, and Mr. Trump has found himself increasingly relying on Mr. Rubio. Unlike Steve Witkoff, the real estate investor who has been used by Mr. Trump as a special envoy to Russia and the Middle East despite having no previous experience with diplomacy, Mr. Rubio has a long record in dealing with the nuances of foreign policy and national security.
Some within the administration once viewed Mr. Rubio, who battled bitterly with Mr. Trump for the 2016 Republican presidential nomination, with skepticism. But Mr. Rubio has shown steadfast loyalty to the president, and has been rewarded with expanded responsibilities.
In addition to his roles as secretary of state and national security adviser, he is also the head of U.S. A.I.D. and the acting archivist at the National Archives.
“From what I’ve been hearing, I think everyone inside of the administration has been very happy with him, and some even pleasantly surprised,” said Leslie Shedd, a fellow at the Atlantic Council’s Eurasia Center who worked on foreign policy issues on Capitol Hill. “They didn’t know what to expect, and he has really been there standing up for the president, and fighting for what the president wants.”
He had begun moving in the direction of Mr. Trump’s “America First” foreign policy before the 2024 election, and since joining the administration has tamped down his harshest criticism of the Russian president as he attempts to make a deal.
“We can’t end the war without talking to Mr. Putin,” he said in congressional testimony in May, during which he declined to repeat his accusation that the Russian president is a war criminal.
“War crimes have been committed, no doubt,” he said. “And who is responsible for that? There will be a time and place for that accountability. But right now, the job is to end the war.”
During their meetings with Mr. Trump, European leaders have been seeking “Article 5-like” protections for Ukraine, a reference to the commitment in NATO’s charter that an attack on one member is considered an attack on all of them.
Such a commitment could mean that if European allies send troops to Ukraine and are attacked, then the U.S. would send troops as well, a move Mr. Trump has ruled out.
European leaders will also be particularly attuned to the Russian strategy of slowing down negotiations while trying to improve its negotiating position on the battlefield. Ms. Fix said European leaders would remember Russia employing the tactic during the Minsk negotiations in 2014 and 2015 over clashes in eastern Ukraine following Mr. Putin’s annexation of Crimea.
“That experience of negotiating with Russia for a really long time is something that they remember from Minsk and how that led nowhere,” she said.
Whatever deal is struck, European leaders would have to sell it at home as well. Mr. Trump suggested this week on Fox News that France, Germany and Britain were prepared to put “boots on the ground” as part of a security guarantee.
But residents in those countries have concerns about sending soldiers to die in a foreign land, much as Americans do.
“Even if Europeans say we are ready to do that, I would expect that for some countries, it might actually not be that easy, especially if Trump’s security guarantees, or whatever Rubio proposes, is not as strong as Europeans would like to see it,” Ms. Fix said.