Why did Liam Fox's position become untenable? One minute he seems determined to battle on -- then, he is gone.
Clearly he was being undermined by the constant drip-drip of new allegations or inferences, with different elements of the story emerging every day. That made it hard for the (now former) defence secretary to get on with his job, which creates a sense of dysfunction and drift in government. The longer that goes on, the likelier it becomes that the prime minister's command of the situation will come under question. That is clearly the turn events had taken in the last 24 hours.
When there is scandal in the air, Downing Street wants to be able to say it is awaiting the outcome of investigations -- or lines to that effect. The PM needs a way of asserting control. That was impossible this week because it kept getting harder to know what was under investigation.
Fox's contact with Werrity? Werrity's contacts in the international defence industry? Werrity's financial affairs and how they might have been intertwined with his work alongside Fox? The international political/ideological component of Werrity's lobbying activities and connections? Printing dodgy business cards is one thing, running a shadow foreign policy is something else entirely.
In other words, the whole thing had spiralled beyond possible breaches of the ministerial code. Questions were being raised about whether Fox's independence and integrity as the senior politician in charge of UK defence might have been compromised.
When Werrity looked like an eccentric accompaniment to Fox's entourage the worst offence that could be levelled against the defence secretary was breach of protocol. Hence senior civil servants were invited to investigate. It was all just a matter of process. If it turned out some Whitehall niceties had been overlooked, well, that would not have been a sackable offence.
But the issue that Fox could never quite get clear of was the suggestion that he ought to have been especially careful given the specific sensitivities of his portfolio. His friends consistently sneered at the idea that there was a "national security" dimension to the allegations. But ultimately, that, I think, is what has finished him. There was simply too high a volume of mystery around Werrity -- and what personal agenda he might have had -- for people, including ultimately the Prime Minister, to be relaxed about him swanning in and out of the MoD and taking intimate foreign trips with the Defence Secretary.
Cameron wanted to hang on to Fox for long enough to show to the right wing of his party that he wasn't itching to despatch one of their standard bearers from the cabinet. But the perception of a labyrinthine trail of sleaze was starting to damage the PM. Cameron is also generally reluctant to reshuffle his cabinet -- so much so as to be almost allergic to the concept. Now, of course, he has no choice.