Praising the strength of America's oldest alliance, US President Donald Trump welcomed French President Emmanuel Macron to the White House with a pomp-filled ceremony on the South Lawn.
The ceremony opened a day of talks on the future of the Iran nuclear deal and the crisis in Syria.
Mr Trump said the partnership he forged with Mr Macron at the start of his presidency was a testament to the "enduring friendship that binds our two nations".
He thanked the French leader for his "steadfast partnership" during the recent missile strike in response to the chemical attack in Syria.
Mr Macron told Mr Trump that together the US and France would defeat terrorism, curtail weapons of mass destruction in North Korea and Iran and act together on behalf of the planet, a reference to Mr Macron's work to revive a US role in the Paris climate accord.
"History is calling us. It is urging our people to find the fortitude that has guided us in the most difficult of times," Mr Macron said. "France and with it, Europe, and the United States have an appointment with history."
The pageantry of Mr Macron's official state visit, the first of the Trump presidency, comes Tuesday night with a lavish state dinner at the White House. About 150 guests are expected to dine on rack of lamb and nectarine tart and enjoy an after-dinner performance by the Washington National Opera.
Monday night was more relaxed, featuring a helicopter tour of Washington landmarks and a trip to the Potomac River home of George Washington with their wives for dinner.
The presidents and their spouses hopped on a helicopter bound for Mount Vernon, Washington's historic riverside home, for a private dinner one night before the leaders sit down for talks on a weighty agenda including security, trade and the Iran nuclear deal.
"This is a great honour and I think a very important state visit given the moment of our current environment," Mr Macron said on Monday after his plane landed at a US military base near Washington.
Mr Macron's pomp-filled three-day state visit to Washington underscores the importance that both sides attach to the relationship: Mr Macron, who calls Mr Trump often, has emerged as something of a "Trump whisperer" at a time when the American president's relationships with other European leaders are more strained.
Mr Trump, who attaches great importance to the optics of pageantry and ceremony, chose to honour Mr Macron with the first state visit of his administration as he woos the French president.
For all their camaraderie, Mr Macron and Mr Trump disagree on some fundamental issues, including the multinational nuclear deal, which is aimed at restricting Iran's development of nuclear weapons.
Mr Trump, sceptical of the pact's effectiveness, has been eager to pull out as a May 12 deadline nears. Mr Macron says he is not satisfied with the situation in Iran and thinks the agreement is imperfect, but he has argued for the US sticking with the deal on the grounds that there is not yet a Plan B.
The Trumps and Macrons helped plant a tree on the White House grounds together before boarding Mr Trump's Marine One helicopter for a scenic tour of monuments built in the capital city designed by French-born Pierre L'Enfant as they flew south to Mount Vernon, the first US president's home along the Potomac River.
The young oak is an environmentally friendly gift to the White House from Mr Macron, and one that also bears historical significance. It sprouted at a First World War site in France, the Battle of Belleau Wood, that became part of US Marine Corps lore.
After mr Trump's helicopter landed at Mount Vernon, the two presidents, each holding his wife's hand, walked a short distance and posed for pictures before they boarded golf carts that ferried them to the front door of Washington's plantation house.
The couples were led on a brief outdoor tour before they entered the pale yellow building for dinner of Dover sole, pasta stuffed with lemon ricotta, and chocolate souffle and cherry vanilla ice cream.
Mr Trump declared the dinner "really fantastic" before returning to the White House.
He ended his first year in office without receiving a foreign leader on a state visit, the first president in nearly 100 years to fail to do so.
He was Mr Macron's guest last July at the annual Bastille Day military parade in the centre of Paris. mr Macron and his wife also took mr Trump and America's first lady on a tour of Napoleon's tomb and whisked them up the Eiffel Tower for dinner overlooking the City of Light.
Mm Macron was welcomed back to the White House on Tuesday with a traditional arrival ceremony featuring nearly 500 members of the US military and a booming 21-gun salute. The state visit also offers Mr Macron his first Oval Office sit-down with Mr Trump and a joint White House news conference. There's also a State Department lunch hosted by Vice President Mike Pence.
The French president's White House day will be capped Tuesday night with a state dinner, the highest social tribute a president bestows on an ally and partner.
Melania Trump played an active role in every detail of the visit, said White House spokeswoman Sarah Huckabee Sanders.
The first lady settled on a state dinner menu for about 150 guests. On Monday, she released details of the glitzy affair being planned to dazzle Mr Macron and his wife, Brigitte.
Dinner will be served in the State Dining Room, which will feature more than 2,500 stems of white sweet pea flowers and nearly 1,000 stems of white lilac.
Separately, more than 1,200 branches of cherry blossoms will adorn the majestic Cross Hall.
The first lady opted for a cream-and-gold colour scheme, and will use a mix of china services from the presidencies of Bill Clinton and George W Bush.
State dinner tickets are highly sought after by Washington's political and business elite.
A few of those expected to attend are: Christine Lagarde, head of the International Monetary Fund and a former top French government official; House Speaker Paul Ryan and treasury secretary Steven Mnuchin and his wife, Louise Linton.