LONDON — New British Prime Minister Keir Starmer doesn’t get to take a breather. After a draining six-week election campaign, the center-left politician must get straight to work assembling his government, tackling a mountain of domestic problems and putting his stamp on the U.K.’s relations with the rest of the world.

It’s a daunting list for a new leader who has never served in, much less led, a government. But Starmer insisted that he is up to the challenge of heading the U.K. in a world that is “a more volatile place” than it has been for many years.

Like someone moving into a new home with their IKEA furniture, Starmer’s first task was to assemble a Cabinet.

Starmer began putting together his government soon after he walked through the door of 10 Downing St. on Friday afternoon following his landslide election victory. He has a plethora of lawmakers to choose from – his Labour Party won more than 400 seats in Thursday’s election, almost two-thirds of the 650 in the House of Commons.

Key players in the new administration include Treasury chief Rachel Reeves – a former Bank of England economist and the first woman to hold that job – who will liaise with international financial institutions.

The new foreign secretary, Britain’s top diplomat, is David Lammy, a Harvard Law School graduate who vowed to “reconnect Britain for our security and prosperity at home.”

Starmer has said his message to the world from the U.K. is: “We’re back.” He wants Britain to take a larger role on the global stage after years of soured relations with Europe over Brexit and the inward-looking U.K. political soap opera that followed.

In the hours after taking office, the new prime minister held a string of calls with world leaders, including U.S. President Joe Biden, Indian Prime Minister Narendra Modi, European Union leader Ursula von der Leyen and Ukraine’s Volodymyr Zelenskyy. He also dispatched Lammy on a weekend trip to Germany, Poland and Sweden.

Starmer takes office at a time of multiple crises — including the war in Ukraine and the Middle East, a surging far right, and a muscle-flexing China — that will test the cool head he honed in his former job as Britain’s chief prosecutor.

Malcolm Chalmers, deputy director general of the Royal United Services Institute, a military think-tank, said that with France facing a far-right surge in legislative elections and the U.S. embroiled in a polarizing, high-stakes presidential election, Britain “has the most stable government of all the major Western democracies.”

“It therefore has the opportunity, and responsibility, to help steady the ship of Western unity at a time of exceptional political fluidity,” he said.

Starmer is set to make a high-profile international debut by flying to Washington Tuesday for NATO’s 75th-anniversary summit. The gathering is overshadowed by questions about Biden’s fitness and the uncertain outcome of the November U.S. presidential election.

Starmer has said there will be no change to Britain’s staunch support for Ukraine, and has pledged to increase U.K. military spending to 2.5% of GDP — though he hasn’t put a date on it.

“The decision for Starmer is how much to try to persuade the U.S. –- as well as other wavering members of NATO — to remain a defender of Ukraine, on the grounds not just of sovereignty but European security,” said Bronwen Maddox, director of international affairs think-tank Chatham House.

Starmer also faces divisions within his party over Israel’s war against Hamas. Several Labour lawmakers were defeated in the election by challenges from pro-Palestinian independents angered by the party’s initial refusal to call unambiguously for a cease-fire in the Gaza conflict.

On July 18, Starmer will host leaders from across Europe at England’s Blenheim Palace for a meeting of the 47-nation European Political Community. It’s a chance to begin to mend ties with the U.K.’s neighbors in Europe, which have frayed and soured since the U.K. left the European Union in 2020.

Starmer says he wants to reduce some of the post-Brexit barriers for people and goods that have hobbled relations between Britain and the bloc. He insists, though that he won’t reverse Brexit, or rejoin the EU’s single market and customs union.

Critics say that shows a lack of principle, but supporters say it respects Britons’ desire not to reopen the divisive Brexit debate.

Starmer also faces a daunting range of domestic challenges, underlaid by the widespread sense that many facets of Britain’s public life have broken down during 14 years of Conservative rule

Stagnant growth and low productivity restrain the economy while millions of people struggle with the cost of living. The country’s aging infrastructure is creaking and the state-funded National Health Service is at breaking point, with long waiting lists for treatment, a situation that has been worsened by a long-running series of doctors’ strikes. Ending that dispute will be a priority, but Starmer has vowed to keep a tight lid on spending and won’t want to agree to the big salary increases the doctors are seeking.

Starmer promises to take control of migration and stop people making dangerous journeys across the English Channel in small boats, but he is scrapping the Conservative government’s controversial plan to send asylum-seekers on a one-way trip to Rwanda — declaring Saturday that the Rwanda deal was “dead and buried.” The U.K. has paid Rwanda hundreds of millions of pounds (dollars) as part of the deal without a single flight taking off.

He’s also expected to emphasize Britain’s commitment to fighting climate change after a series of backward steps under the Conservatives.

The new government’s legislative plans for the next year will be set out on July 17 in a speech delivered by King Charles III at the ceremonial State Opening of Parliament.

“A lot of people are feeling a sense of potential transformation,” said Lise Butler, senior lecturer in modern history at City University of London. “I think that there’s going to be elevated expectations for Starmer, which are going to be difficult to meet.”

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Associated Press video journalist Kwiyeon Ha contributed to this story.