Mr Arbuthnot, despite being a dyed-in-the-wool Tory, was completely non-partisan as a committee man and a thorn in the side of Downing Street as he led a series of inquiries into Afghanistan, the strategic defence review and many other areas, which left ministers looking foolish. The view generally is that his successor has a lot to live up to and will have the job of holding the government to account on the next stage of defence reforms and the renewal of Trident.
The way Commons rules work is that the chairmanship must go to the Conservatives but can be voted on by MPs from all parties. There are eight candidates, all of whom have their merits but none of whom is perfect.
The man generally seen as the frontrunner, therefore unlikely to win, is Broadland MP Keith Simpson, who rather unfairly has the fatal tag of being the “Downing Street candidate” because he is the parliamentary private secretary to Foreign Secretary William Hague. Mr Simpson is, rather unusually, qualified for the job. In fact he taught four of his opponents military history at Sandhurst and was an academic in war studies at Kings College, London. He has written to MPs promising to be even harder on ministers than Arbuthnot. It may not be enough.
For many the contest is actually between the “visionary” – former diplomat and maverick Scottish Penrith and The Border MP Rory Stewart – and the “pain in the neck” – New Forest MP Julian Lewis. Both are disliked by Downing Street.
Of these two, Dr Lewis is the most likely to win because he has made life so difficult for David Cameron, which should guarantee the Labour and disaffected Tory back-bencher vote. But he will need to talk a lot less about his favourite topic: renewing Trident.
But still in the contest is the former colonel, Bob Stewart, who famously served in Bosnia. He is eminently qualified, knows defence inside out and is the choice of the committee itself, but is rather hampered by declaring his hand too early and having a rather bluff image.
The other runners and riders are Canterbury MP Julian Brazier, former officers Tobias Ellwood, who may be a non-runner due to lack of nomination signatures, and Crispin Blunt; and James Gray, the man who famously lasted only a week as shadow Scottish secretary before having to resign.