Keir Starmer faces his greatest test in foreign policy – one that could define his premiership

In 1950, amid some wild talk in America about how far the US might go to prevail in the Korean War, Clement Attlee left Number 10 in haste to fly to Washington to caution Harry Truman about the use of nuclear weapons. In 1990, when Saddam Hussein ordered Iraq's army to invade Kuwait, Margaret Thatcher sought to stiffen the spine of George H Bush by telling that US president: "Remember, George, this is no time to go wobbly.

- Andrew Rawnsley

British prime ministers have sometimes urged muscular action on the White House; at other times they have been the voice of circumspection. On America joining Israel's war against Iran, Sir Keir Starmer's role model is not the gung-ho Thatcher, but the restraining Attlee. In the days since Israel launched the conflict, he has relentlessly urged de-escalation while expressing the hope that the US will not pile in against the regime in Tehran when it looks temptingly vulnerable to Washington's hawks.

Is this having any traction with Donald Trump? The US president seems to be charmed by Sir Keir and regularly says he likes him. But the prime minister will be kidding himself if he thinks he has all that much leverage over the decision-making processes in Trump world. Number 10 put it about that their man played a significant role when the leaders of the G7 had a working dinner in the Canadian Rocky Mountains. Sitting next to the US president, Sir Keir believed he'd won a victory by helping to persuade the American to put his name to a joint statement calling on Israel and Iran to pull back from an all-out conflict. He then told the media that there was "no doubt in my mind" that the US president did not want the war to escalate.