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These graduated responses, though close to the border and claiming casualties, are still considered within the bounds of acceptable behaviour. The deeper into each other’s territory each side conducts operations, and the higher the profile of the targets they choose, however, the greater the risk that one side over-reaches and the limited conflict spills out into all-out war. But we remain some way from that yet in Lebanon.

The US government has employed the same constrained approach against Iranian-backed Shi’a militias in Syria and Iraq, targeting static positions manned by militia members and advisers. The killing of a militia leader in Baghdad in a US airstrike a week ago was a clear indication from Washington that militias now need to adjust the cost-benefit analysis of future attacks against US forces.

It may well be though, that the Yemen front proves the most challenging. Following an attack on a US warship in 2016, the US issued a successful deterrent message to the Houthis by destroying three of their radar sites with Tomahawk missiles. But the Houthis are more capable and more experienced now than they were then.

Though Washington could deliver a heavy response to the recent attacks on vessels in the Red Sea easily at any time, the Americans are keenly aware that the Houthis still hold crew members from detained vessels, and that they could respond in unconventional ways by launching more missiles at Israel, targeting US allies such Saudi Arabia and the UAE, or killing detained merchant sailors. This could risk further destabilisation in the region.

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Having publicly released the letter calling for an end on attacks and the release of the detained vessels, though, the US government cannot now fail to follow through on its threat. As US president Barack Obama learned in 2013, when you draw a line in the Middle East, you need to follow it.

The multiple conflict fronts that have opened up across the Middle East since October 7 present a range of challenges, both singularly and collectively. Though there is little to indicate metastasis will ultimately come to pass, as Biden fears, each threat actor has their own interests distinct from those of Iran that will affect decision-making.

They do, however share the common strategic aim of destabilising the region for as long as Israel remains in Gaza. Until then, it is likely that we will see a continuation of these limited attacks against Israeli assets and those of the US and perhaps their partners. The attacks will be calibrated so that they don’t transgress the unwritten rules of retaliation in the region. But as with all unwritten rules, there is always lots of room for misinterpretation.

Dr Rodger Shanahan is a Middle East analyst, former army officer and the author of Clans, Parties and Clerics: the Shi’a of Lebanon.

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