Starmer, who was meeting Trump at the White House for the first time since the US leader started his second term, turned on the charm, saying peace in Ukraine had only become possible because of Trump.
In the Oval Office on Thursday, Starmer handed a letter of invitation from King Charles for a state visit. Trump accepted. A date hasn't been set.
But underlying differences between the allies remained on the table for the private conversation that followed. They include transatlantic frictions over US-Russia talks aimed at ending the Ukraine war.
Before the meeting, Starmer had argued that there could be no long-term peace in Ukraine without firm US security guarantees - an argument Trump all but dismissed.
"We are a backstop because we'll be over there, we'll be working," as a result of the economic partnership, Trump said. "We're going to have a lot of people over there."
Starmer is the latest European leader to meet Trump after French President Emmanuel Macron came to the White House on Monday for a friendly encounter that nonetheless displayed stark differences about Russia's war with Ukraine and the US push for a quick ceasefire.
"It's moving along pretty rapidly," Trump said of ceasefire diplomacy, expressing optimism that Russian President Vladimir Putin would keep his end of any bargain. "It will either be fairly soon or it won't be at all."
"We have to get it right," said Starmer at a joint press conference with Trump. "It can't be peace that rewards the aggressor."
Trump, who came into office on January 20, has shocked traditional US allies in Europe by drawing closer to Putin, calling Ukrainian President Volodymyr Zelenskiy a "dictator," and demanding payback for US financial support for Kyiv.
On Thursday, Trump distanced himself from the dictator comment and said he gets along with the Ukrainian leader.
Zelenskiy is expected to be in Washington on Friday to sign an agreement with Trump on rare earth minerals, a deal the Ukrainian leader said would hinge on further US aid.
Trump portrays the deal as a way to recoup American money that has been spent to support Ukraine. It includes no specific security guarantees for Ukraine.
Starmer has signalled that Britain will increase defence spending and is expected to try to reassure the US president that Europe will provide support and security guarantees to Kyiv if peace talks with Russia are successful.
A senior Trump administration official told reporters they were pleased with Starmer's pledges to increase defence spending.
Putin on Thursday warned "Western elites" against trying to sabotage a potential rapprochement between Russia and the United States, saying Moscow would use its diplomats and intelligence services to thwart such efforts.
The remarks were an apparent reference to the European Union and Britain.
Trump has shattered foreign policy and domestic policy norms since the start of his second term, rattling allies by advocating for US ownership of the Gaza Strip and by promising trade tariffs on US friends and foes alike.
Like Macron, Starmer will argue that a rushed peace deal with Russia, without the participation of Ukraine or European nations, might lead to further instability in Europe, which would not be good for the United States.
Starmer has said he is open to British troops providing security guarantees to Ukraine but only alongside other European nations and with "the right conditions in place."
European countries are concerned about the high level of conflict in Ukraine now, the US official said, while a ceasefire would give them more comfort that their role is more about peacekeeping than deterring active conflict.
