Britain's first prime minister, Horace Walpole, once wrote that no great country was ever saved by good men because good men will not go to the lengths that may be necessary.
Lessons from history are just that, lessons; but the primary rule of politics remains the same as it was when Walpole practised his arts: ruthlessness lies at the heart of leadership.
The dictum is worth remembering, given the near-grief exhibited by some around the Commons since news first broke late on Tuesday that David Miliband was quitting British politics.
His decision to leave, along with his American-born wife, Louise, and two adopted American children, to lead a New York-based international charity ends a two-year melodrama at the heart of Labour. Undoubtedly talented, the elder Miliband has struggled to define himself since he was beaten by his brother Ed in late 2010.
During the battle, one of those involved in the leadership campaign shrewdly observed that the siblings were not “just fighting for the leadership, but for the right to the surname”.
Since then, the younger brother has become “Miliband”; the elder, meanwhile, has struggled to create a new existence, where his every move is interpreted by its implications for the other.
Certainly his departure means that he is convinced that his brother will not face a challenge before the next election – something that could not have been entirely ruled out last year given Labour’s fratricidal nature.
Miliband snr lost an election he should have won: the Labour organisation backed him; so did most MPs – he failed because the trade union bloc vote swung behind his brother.
However, he should not have lost: four more MPs would have made the difference, but Miliband snr was slow to call them, even slower to charm those needing persuasion.
Five years ago, Miliband snr chose late July to publish a piece in the Guardian outlining his vision for Labour's future, but he pointedly made no mention of Gordon Brown, who had just gone on holidays.
It was interpreted, rightly, as a challenge, but no steel was produced. More than a few of the MPs he would later depend on saw it for what it was, but it was difficult to know what rankled most – the disloyalty, or the failure to follow through.
In June 2009, he could have acted to end the misery of Brown's leadership after the then works and pensions secretary of state, James Purnell, quit in fury because Brown would not let him run his office.
However, Purnell left believing that he would not be alone, that Miliband snr would be behind him. So, too, would others, leading to the inevitable implosion of Brown’s tenure.
But he was alone. Miliband snr walked away from the guns, satisfying neither side when a declaration of support for Brown was slow in coming and weak when it arrived.
Back then, most of them knew Brown was leading them to destruction – the Purnell exit was the last opportunity for salvation and it was lost, principally, because Miliband snr buckled. The severely damaged relations between the two brothers – driven by the sense from Miliband snr’s camp that their champion was “entitled” to the top prize – have not improved.
Today, he is added to a long list of “might have been” prime ministers. His brother may or may not become one, but it is a chance he would not have had if David Miliband had the steel required for high office.