It was 28 minutes into Joe Biden’s press conference when the gaffe came – and it was a jaw-dropper. Especially if you were watching in Kyiv or the Kremlin.

The US President appeared to say that, if Vladimir Putin were to make only a “minor incursion” into Ukraine, then Russia would not face devastating sanctions.

His exact words were: “Russia will be held accountable if it invades. And it depends on what it does. It’s one thing if it’s a minor incursion and we (in Nato) end up having to fight about what to do and what to not do, etcetera.

“But if they actually do what they’re capable of doing, it is going to be a disaster for Russia if they further invade Ukraine.”

The comment whizzed around the world and, as the implications of what he had said sank in, a journalist read it back to him.

Was Mr Biden giving “permission” for a “minor incursion” by Russia into Ukraine, the journalist asked.

Mr Biden laughed and said: “Good question. It did sound (like) that, didn’t it…”

In Kyiv, there were accusations Mr Biden was giving a “green light” to the Kremlin, and anger that he would publicly make a distinction between different levels of Russian attack.

According to the Ukrainians, that was a particularly dangerous thing to do as Mr Putin’s preferred option is believed to be a limited incursion.

It led to suspicions a deal was being done behind Ukraine’s back. Indeed, the fear in Kyiv was that Mr Biden had not so much been wrong, but that he had inadvertently told the truth.

Whatever he meant, it was an example of Mr Biden saying too much, and talking loosely, a failing for which he has form.

In his first press conference for 78 days, the President was perhaps seeking to demonstrate his command of detail, ultimately speaking for almost two hours.

They said what Mr Biden had been talking about was the divisions in Nato over how to respond to Russian aggression. It was also suggested that by “minor incursion” he had meant Russian cyber attacks, rather than a small military invasion.

Following the presidential misstep it was left to Jen Psaki, the White House press secretary, to state the official US position.

“If any Russian military forces move across the Ukrainian border, that’s a renewed invasion, and it will be met with a swift, severe, and united response,” she said.

A White House national security official added: “He (Mr Biden) was referring to the difference between military and non-military/para-military/cyber action by the Russians.”

Within the hour it was also announced that Anthony Blinken, the US Secretary of State, would make a speech about Ukraine the following morning in Berlin.

Mr Blinken was expected to deliver a much more forceful commitment to the defence of Ukraine than his boss.

For Mr Biden it was the latest gaffe on foreign policy.

In October, his officials had to calm the waters after he suggested the US would come to Taiwan’s defence in the event of an attack by China, appearing to shift Washington’s delicate longtime policy of “strategic ambiguity”.

On Wednesday, when he was first asked about the Ukraine crisis, Mr Biden said: “He (Mr Putin) has never seen sanctions like the ones I’ve promised if he moves (into Ukraine).”

That was what he was expected to say, and his advisers must have wished he stopped there. Instead, Mr Biden then embarked on a lengthy analysis of what he thinks is going on inside Mr Putin’s head – a notoriously difficult thing to predict.

He went into great depth speculating on what Mr Putin might believe about a variety of subjects, including fires on the Russian tundra and nuclear war.

If he was watching – it was the middle of the night in Moscow – Mr Putin must have been rather puzzled by it all.
But he will have concluded that the door to Ukraine may be ajar.