British reluctance to be a conduit for a US military buildup in the Gulf seems to have taken Washington by surprise but it demonstrates the lessons of Iraq are deeply etched in Whitehall. This time the UK is making clear it will not write blank cheques and will take its own decisions on what is legal and what is not.
The argument does not reflect a difference of opinion over the utility of going to war as a means of stopping Iran's nuclear programme. US military commanders, like their British counterparts, believe that any such venture is likely to backfire in current circumstances, persuading the Iranian leadership that it has no choice but to make nuclear weapons as fast as possible.
Circumstances will change, however. A new diplomatic push is planned for after the US elections, offering sanctions relief in return for limits on Iranian uranium enrichment. But if that initiative fails, tensions will rise, and rash moves with unintended consequences will become more of a risk. The military calculus facing the next US president will present itself in ever more shades of grey, as will the legalities.
The most immediate unknown is what Israel will do if diplomacy fails. US commanders insist publicly and privately they are in the dark about Binyamin Netanyahu's thinking on a military strike aimed at setting back Iran's progress. The betting in the Pentagon this week is 60-40 against an Israeli attack, but the odds change every day.
If Israel strikes and Iran responds with an attack against the US or its Gulf allies, then Washington would undoubtedly retaliate. If the Iranian response is tightly focused on Israel, and roughly proportionate, the American reaction is harder to predict. It depend to some extent on who will be in the Oval Office at the time. And again, the legalities of any action would depend on the details.
The Iranians will be making their own calculations, but it is a safe bet the country's supreme leader, Ali Khamenei, will not jeopardise the regime's survival. An all-out US assault would do just that, and awareness of that risk underlies everything Tehran does, but as the stakes rise the law of unintended consequences begins to take over.
As the sanctions screw tightens on Iran, the regimes or elements within it could miscalculate. Iran is finding it harder and harder to sell its oil and, by some estimates, will run out of storage space for it in January, potentially forcing it to turn off the pumps and thereby degrade its own infrastructure.
US military officials speculate that if Tehran, or more specifically the Revolutionary Guards, decide to strike back against a sanctions regime Iran sees as an act of war, they would do so in ways they hope would be unattributable, by secretly mining the shipping lanes or triggering a vast oil spill in the Strait of Hormuz. That would make the Strait impassable to shipping and threaten desalination plants which are essential to life in the Arab Gulf states.
Or the Iranian navy could start boarding and inspecting shipping. Each move would invite a counter-move from the US Fifth Fleet, which has been preparing for each contingency, conducting its biggest ever minesweeping exercise in the Gulf last month and beefing up its capacity to handle oil slicks.
American and Iranian warships already operate in close proximity in the Gulf, and although the two countries have no diplomatic relations, naval etiquette requires their sailors to talk to each other from bridge to bridge every day. Such polite encounters could easily turn sour and run out of control if tempers heat up.
Alternatively, the leadership in Tehran could gamble by raising the level to which it enriches uranium in the hope of forcing western concessions at the negotiating table. Iranian parliamentary officials have suggested the current top level of 20% could be raised to 60%, close to weapons grade. That could cross not just Israeli red lines, but American red lines too.
Barack Obama has said he would act to stop Iran building a nuclear weapon, while Mitt Romney said he would prevent Iran's "nuclear-weapon capability", leaving his definition vague.
As the scenarios proliferate, so do the ramifications in international law, focusing on what constitutes legitimate self-defence. Little wonder the British government is keen to retain its own counsel.
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