Washington today unveils what its own treasury secretary called an "economic D-Day" - the harshest sanctions package against Iran so far. But the real message is not addressed to Tehran. It is addressed to everyone else.

US treasury secretary Scott Bessent was due to face reporters this evening and explain the details. Earlier, in a Financial Times column, the tone was set without hedging: "At dawn an economic D-Day begins - the largest financial offensive ever launched against a single adversary."

President Donald Trump announced "economic war and unprecedented isolation" on 19 August, warning that the consequences would hit every country whose banks, companies, airports or state institutions assist the Iranian economy. A day later Bessent went further still, describing the operation on American television as "the largest coordinated economic isolation in world history".

The sentence that says everything is older than all of us: "You are either with us or against us." The Balkans know this sentence by heart - and know what usually follows it.

The elephant in the room is called China

The question is not what Europe will do. The question is what Beijing will do. According to agency reports, China buys over 80 percent of Iran's seaborne oil exports. Without a Chinese buyer, the sanctions are paper. With a Chinese buyer, the sanctions become a test of how far Washington is willing to go.

Asked directly whether the American campaign would also target Beijing, Bessent did not answer openly. He said some conversations are best held privately, then added that China gets around 50 percent of its energy from the Gulf and therefore has an interest in the Strait of Hormuz staying open. The message ended with an almost polite formulation - it would be good if China "joined the programme".

That is not diplomacy. That is a bill delivered with a smile.

Iran does not speak with one voice

The ultimatum simultaneously exposed the split inside Tehran itself. Iranian president Masoud Pezeshkian said on Friday it would be better for the war to end now, while Iran is, as he put it, in "a position of strength and dignity".

Almost at the same hour, the opposite message came from the military top. Chief of the General Staff Major General Ali Abdollahi warned Iran would respond "painfully, punitively and devastatingly", and that its forces are ready on land, at sea, in the air, in air defence and in cyberspace. Mohsen Rezaei, the new secretary of the Supreme National Security Council and a former Revolutionary Guard commander, went furthest - telling neighbouring countries that joining the American campaign could be treated as a hostile act.

The Gulf between two fires

The Gulf states are effectively stuck. Washington tells them they must pick a side. Tehran tells them what the choice costs. And Iranian threats to strike energy infrastructure are not only rhetoric - Saudi energy infrastructure was hit in the April attacks, including the strategic east-west pipeline that lets Saudi Arabia export while bypassing Hormuz.

Beijing has no reason to enter an American-Iranian war directly. But it has a strong interest in Iran not being broken outright. The same logic already surfaced with Russia - Chinese foreign minister Wang Yi told EU foreign policy chief Kaja Kallas that China does not want a Russian defeat in Ukraine, because that would let Washington turn its full attention to China.

So everyone knows the game. Beijing will not save Iran, but it will not let it fall either. Washington knows this. And it is announcing the largest financial offensive in history anyway, against an opponent whose biggest customer has no intention of stopping.

Who exactly blinks first in that calculation - that is the one question tonight's press conference will not answer.