The killing of foreigners.

A cynical conclusion? Well, two hundred Palestinian aid workers have been killed in Gaza since the 7 October Hamas attacks on Israel.

They are among more than the 30,000 killed.

Much of the strip has been flattened. Hospitals have been destroyed along with other infrastructure which allows a population to function - mosques, courts, schools.

There is an acute humanitarian crisis across the strip topped by a famine in the north.

But it was the death of foreigners working for the charity World Central Kitchen which prompted the American shift.

One US official said this moment "is an inflection point in this war".

Mr Biden even called for an immediate ceasefire despite the US abstaining in a vote for just that at the UN a week ago.

The White House's official summary of the call between President Biden and Prime Minister Benjamin Netanyahu suggests it was particularly tense.

Mr Biden, we are told, "made clear the need for Israel to announce and implement a series of specific, concrete, and measurable steps to address civilian harm, humanitarian suffering, and the safety of aid workers".

And then the crux: "He made clear that US policy with respect to Gaza will be determined by our assessment of Israel's immediate action on these steps."

That key point was developed by secretary of state Antony Blinken.

"And with regard to our policy in Gaza…" Mr Blinken said, "look, just say this if we don't see the changes that we need to see, there'll be changes in our own policy".

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Of course what no one has explained is what a change in America's policy would actually look like.

There is plenty of chatter about the prospect of America pulling its military supply chain to Israel. But what that sounds like and what it would look like are two different things. Do not expect America to cut Israel off. Mr Biden wouldn't do it. Congress wouldn't allow it.

If America was to limit its supply of weapons to Israel, it could leave the Jewish state in an existential position. Iran and its powerful proxies across the region could exploit a weakness.

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The only leverage America has is a lever Mr Biden can't really pull because it could expose Israel in an existential way.

And it's not as if America can meaningfully dictate how Israel uses weapons it receives from America.

Look again at the language the Americans are using. Mr Biden talked about US policy "with respect to Gaza…". Later in the statement: "President Biden made clear that the US strongly supports Israel in the face of [Iranian] threats."

That's the nub of it. How to rein in Mr Netanyahu with respect to Gaza when the key lever you have to pull is one you really can't. That's Mr Biden's challenge. And it's Mr Netanyahu's advantage.