President Trump and French President Emmanuel Macron plant a tree as first lady Melania Trump and Macron's wife, Brigitte Macron, watch on the South Lawn at the White House on Monday. (Jabin Botsford/The Washington Post)

French President Emmanuel Macron’s long-shot diplomatic mission to persuade President Trump to preserve the Iran nuclear deal began Monday with smiles and, yes, a handshake, which masked the difficult discussions underlying the first state visit Trump has hosted.

The three-day trip is a test of Macron’s year-long effort to cultivate a positive relationship with Trump.

The 40-year-old political newcomer is the envoy for a European effort to address Trump’s main quarrels with the 2015 international pact with Tehran through smaller side deals and diplomatic understandings. Trump loathes the international agreement but has left a sliver of hope that he may not junk it.

Macron has always faced long odds in his effort to convince Trump to stay the course on the Iran deal, but that effort has only become more difficult now that changes to the president’s national security team have replaced advisers supportive of the deal with aides who disdain the pact.

John Bolton is now fully ensconced as Trump’s national security adviser and CIA Director Mike Pompeo will soon be confirmed as secretary of state. Both men are more hawkish toward Iran than their predecessors — H.R. McMaster and Rex Tillerson — and Pompeo has flirted publicly with sponsoring regime change in Tehran.

This leaves Macron with few allies inside the Trump administration and under greater pressure to use this visit to persuade Trump not to abandon the nuclear deal when it next comes up for review, on May 12. The French president seemed eager to embrace the moment when he arrived in Washington on Monday.

“The United States, like France, has a very particular responsibility at a moment that today is ours,” Macron said in brief remarks in French after he arrived at Joint Base Andrews outside Washington.

France was swift to answer Trump’s request for international backing for airstrikes this month in retaliation for a chemical weapons attack on Syrian civilians, helping to launch more than 100 missiles.

In recent weeks — particularly over Syria — Macron has worked overtime to burnish his relationship with Trump.

He rarely gives interviews, but the day before departing for Washington, he granted an audience with Fox News, his American counterpart’s favored channel. During that interview, already seen as a kind of olive branch, Macron went even further to emphasize his apparent similarities with Trump.

“I’m here to serve my people in my country and make it great again, as somebody I know very well could say,” he told Fox’s Chris Wallace in English.

In France, where Trump remains deeply unpopular, these types of gestures have often fallen flat. Earlier this week, for instance, outrage in the French press amplified after Libération, a leading French newspaper, quoted presidential palace sources calling Trump “very intelligent” and “coherent.”

But the common explanation given — most frequently by Macron himself — is that these overtures are a simple question of strategy. “Our relationship with the United States is absolutely critical, in fact,” Macron said earlier this year. “Fundamental. We need it.”

If some in French political circles have expressed concern that Macron may be setting himself up to be the latest associate humiliated — and discredited — by collaborating so closely with Trump, some analysts said the French president has positioned himself well and does not have as much to lose as his critics contend.

“No one can blame him for trying” with regard to the Iran deal, said Benjamin Haddad, a member of Macron’s official presidential delegation and a fellow at the Hudson Institute. “It’s going to be blamed on Trump mostly. At the end of the day, it’s a U.S. decision.”

Even a failure on Iran may not be too damaging for Macron if he leaves the trip still seen as Europe’s best hope of working with Trump.

“At the end of the day, Macron’s main objective is not Iran, it’s Europe,” Haddad said. “If he manages to emerge as the key interlocutor of the U.S., he can leverage that to push his own agenda.”

Trump has appeared eager to use the visit to solidify his relationship with Macron.

On Monday, Trump and first lady Melania Trump showed off the White House, where the two presidents’ extended handshake recalled the first time they met, which onlookers likened to arm wrestling. The two leaders and their wives then participated in a tree planting on the White House lawn that was meant to symbolize the enduring U.S.-French relationship.

They then headed to Mount Vernon, George Washington’s Virginia plantation home, to honor France as the United States’s first ally. Tuesday’s meeting will be capped off with Trump’s first state dinner.

The French president is also addressing a joint meeting of Congress on Wednesday before heading home.

Macron and his wife, Brigitte Macron, were chosen as the first state guests of the Trump White House largely out of gratitude for extravagant hospitality Macron extended to Trump in Paris last year. The timing, three weeks before a deadline on the Iran deal, set up Macron as the best hope for the deal’s supporters.

White House press secretary Sarah Huckabee Sanders would not discuss the elements of any potential agreement to save the nuclear pact, or say whether there is anything Macron could do to salvage it.

“We certainly expect that that will be part of the conversation,” Sanders told reporters, adding later that Trump is “the best negotiator at the table” and is well equipped for the discussions.

“What you do have are two leaders who have a great deal of respect for one another, who have a great friendship,” Sanders said. “Certainly, both have a great deal of interest in doing what is best for their country.”

Asked whether Trump is still leaning toward withdrawing the United States from the Iran deal, Sanders demurred.

“I don’t have any announcements on that front,” she said. “But the president has been extremely clear that he thinks it’s a bad deal. That certainly has not changed.”

Macron is also expected to lobby Trump to keep U.S. forces engaged in Syria longer — he recently said the troops are coming home soon — and to suggest that Trump revisit his decision to withdraw from the Paris climate accord.

Sanders did not rule out some change of heart on those issues.

“The president wants to make good deals for this country,” she said. “And if he feels like he can make a good deal that benefits this country, he’s certainly going to engage in those conversations.”

Macron’s previous efforts to persuade Trump have seldom fared well, and the two presidents are most often opposed on matters of policy. The Trump administration’s decision to recognize Jerusalem as the capital of Israel was one example of Franco-American disagreement.

Macron had tried to lobby Trump not to withdraw from the historic climate change agreement signed in Paris in December 2015 — a hallmark accomplishment of the Obama administration.

“I was elected to represent the citizens of Pittsburgh, not Paris,” Trump said when announcing the U.S. withdrawal last June.

Ashley Parker in Washington and Carol Morello in Toronto contributed to this report.