The US president knows the attack presumed by most to have been the work of its ally Israel violated international law which declares embassies ‘inviolable’.
But the US president also knows any sign of weakness on his part is dangerous, too.
It would only embolden Iran to do its worst and that in turn would provoke Israel to do the same – potentially setting the entire region alight.
So Joe Biden has declared his ironclad support for Israel and raised the prospect of America becoming directly involved if war were to break out between its ally and Iran.
It is exactly the same calculus that led the US president to send two naval carrier groups to the waters off Israel in the wake of the 7 October attacks by Hamas to warn Iran – ‘don’t get involved’.
That move was successful. This time Biden may need to do more.
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Biden: ‘We want to address Iranian threat’
Iran has shown remarkable restraint holding back in this war despite frequent attacks by Israel on its assets and allies in Syria and Lebanon.
It has done so by claiming those attacks were not direct strikes on Iran itself.
The logic is clear. The ayatollahs are weak at home after the biggest uprising against its rule since its revolution and a regional war would be devastating.
But a direct attack on an embassy can’t be overlooked. The Iranians have made that clear.
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