Tony Blair’s support for the US invasion of Iraq in 2003 has long loomed like a spectre over the Labour party.

It was present in 2013 when Ed Miliband as opposition leader voted to block UK military action against the Syrian regime.

And it was there again on Monday when Keir Starmer assured MPs that the government remembered the “mistakes of Iraq” and would always operate on a “lawful basis” and with a “viable thought-through plan”to the crisis erupting in the Middle East.

The implication from the prime minister was clear: he does not think the initial US and Israeli strikes against Iran were either legal or considered. “This government does not believe in regime change from the skies,” he told the Commons.

It was a significant moment. Starmer has spent much of his premiership carefully dancing around Donald Trump – acting (as he sees it) in the UK’s national interest by maintaining close relations with the US president but taking a lot of flak for it at home.

After Trump told the Daily Telegraph on Monday that it had taken the UK “far too long” to allow the US to use its bases to carry out strikes on Iran, the innately cautious prime minister emphasised that the decision was “deliberate” and he stood by it.

The UK had initially denied the US permission to conduct strikes aimed at regime change from British bases including Diego Garcia and RAF Fairford, citing international law.

But after speaking to Trump at the weekend, Starmer relented on Sunday night. He now said the US military could use the bases for “specific and limited defensive purposes”.

It opened him up to charges of yet another U-turn. But the UK’s position on those initial strikes has not changed. Lord Hermer, the attorney-general, warned that allowing the US to launch attacks for the purpose of regime change – which Trump has himself argued for – would have been a potential breach of international law.

It is not the government position that has altered, but the situation on the ground.

Over the past 48 hours Iranian missiles and drones have rained down across the Middle East, putting hundreds of thousands of British citizens in hotels, airports, residences and military bases at risk.

Officials argued the only way to stop the threat to Britons in the region was to destroy Iran’s ballistic missiles before they were launched: that attempting to strike them in flight was like trying to knock an “arrow” out of the sky, while the US could go after the “archer”.

New legal advice from Hermer concluded that US strikes against missile facilities in Iran – including launch sites and storage silos – would, indeed, constitute self-defence.

“I say again, we were not involved in the initial strikes on Iran, and we will not join offensive action now,” Starmer told the Commons on Monday, ahead of making a similar argument to nervous Labour MPs behind closed doors.

“But in the face of Iran’s barrage of missiles and drones, we will protect our people in the region and support the collective self-defence of our allies, because that is our duty to the British people.

“It is the best way to eliminate the urgent threat, to prevent the situation spiralling further and support a return to diplomacy. It is the best way to protect British interests and British lives.”

The impact of the US-Israeli airstrikes will reverberate across the region for months – if not years – to come. Not least because one of the lessons from the Iraq conflict was that once the “shock and awe” stage is done, there has to be a plan for peace and stability.

Starmer does not believe that the US president has one for Iran. Instead, there is deep uncertainty – and in parts of the British government real dread – about what lies ahead.