Firstly, there is not as much precedent for this as some here pretend.
Kitchener was 64 when the Great War began, and though a Field Marshal does not ‘retire’, (and though his status as a ‘serving’ officer ruled him out as Viceroy in 1911) by the time he became Secretary of State for war, he had been fulfilling ‘civilian’ roles for some years – notably as British Agent and Consul-General in Egypt. Moreover, the post of Secretary of State was not then viewed as being an entirely civilian one, and Kitchener’s predecessor had also been a soldier – Colonel Seeley, who returned to the Army and who saw active service on the Western Front.
In any event, what pertained in 1914 is not really a guide to what is acceptable today. (When war broke out, it was only ten years after a member of the House of Lords had been Prime Minister!)
As to Ismay, his role was explicitely military – as Churchill’s principal link between Churchill and the Chiefs of Staff Committee. He did not take up an overtly political appointment until 1951 – five years after his retirement from the Army.
Secondly, and more importantly, Dannatt’s voice would carry far more weight were he to speak simply as former CGS than it will if he is seen as a Government or Opposition spokesperson.
The issue of properly equipping the Armed Forces for operations in Afghanistan is diminished if it is reduced to a partisan, party political one, and Dannatt’s objections will be more easily dismissed if he himself can simply be dismissed as ‘just another Tory Politician’.
And Dannatt’s recent statements are already being dismissed on the grounds that, as a closet Tory and putative Tory appointee, he was being fed a line by the Tory party, somewhat diminishing the credibility those remarks had when the appeared to come from a professional, apolitical head of the Army.
And what happens when (as will undoubtedly happen) the Tories impose defence cuts of their own, when Dannatt (under a party whip, and perhaps even governed by collective Cabinet responsibility) will be expected to trumpet them as being a good thing.
He will look as morally compromised as Lord Garden did when he was trotted out to justify Lib Dem lunacy on cancelling Typhoon, etc. and he will look just as disloyal to the interests of the service that he once commanded.
This is a sad development, as it makes it more easy to dismiss genuine concerns about equipment for the Armed Forces as cynical, politically-inspired and party-political manoeuvring.
This looks like a bit of an own goal, to be honest.