Having failed the biggest test of his career -- the referendum on Britain's EU membership
-- he had no option but to go quickly and quietly. As all commentators say on occasions like this, "All political careers end in failure."
But very few end in such spectacular failure. The full story of Cameron's miscalculation offers a universal lesson in precisely how not to handle a divided political party -- a lesson that disgruntled senior Republicans should heed in the age of Donald Trump.
One of the iron laws of politics is that when you win, you have more power than you believe and less time than you think.
Electoral victories are said to give you a mandate, but what they really give you is a license: permission to lead, to impose your authority.
Cameron either failed to understand this or, worse, was scared to act. Whatever the underlying motivation, he failed to confront his own party over the European Union question and sowed the seeds of his own eventual defeat. It may seem counterintuitive, but provoking and winning a fight within your own party is the only foundation for sustained political success.
History is often kinder to politicians than their contemporaries are. Hindsight lends a golden glow to the past, and retrospective views make clear exactly how difficult, deep-seated and intractable certain problems are.
A point which is rarely made is how successful David Cameron was as a politician. He took over the Conservative Party when it was at a historically low ebb and had suffered its worst run of defeats in the democratic era: three elections in a row in which it had been held below 200 seats in Parliament.
Cameron took his party back into government -- albeit a coalition -- in his first attempt. After that, he didn't look back and won his next three electoral tests -- a referendum on electoral reform, a referendum on Scottish independence and a second general election -- which gave his party their first majority for nearly a quarter of a century.
In opposition during the Tony Blair years and after, the Conservative Party had become obsessed with Europe to a degree that made it seem eccentric and unfit to govern. By taking them back into government, Cameron had earned the right to not just ignore that strain of conservative thinking, but to isolate and ignore it.
Instead, he appeased it.
In the short term, avoiding conflict often seems the best option. But in the long term, it simply feeds the beast. A fight avoided is a fight deferred, and leaders should take the fight when they are strongest.
And it's this which the Republicans, so unsure of what to do about Donald Trump
, should remember.
Trump's candidacy is the consequence of deferring bitter and difficult internal party conflict over profoundly divisive cultural and cultural issues. To put it bluntly, in an increasingly diverse and educated America, there is no long-term return to presidential election success for the GOP without coming to terms with racial diversity, Hispanic immigration and women's rights, including abortion.
These are all hot-button issues and all avoided by those in the Republican hierarchy who want a quiet life. But if they want to understand why conflict deferred is defeat ensured, they need not look as far as the UK. Just consider the Democrats, whose adherence to the old FDR/New Deal coalition delivered just one president in the six elections between 1968 and 1988.
Just as victory gives authority to make political changes, defeat -- when properly handled -- can become a spur to reform. The key is to define the defeat quickly, clearly and repeatedly. For the Republicans, a defeat for Trump would be just such an opportunity.
Any candidate seeking to be a contender in 2020 would need to have a plausible analysis of Trump's rise and fall. But most importantly, he or she would need a plan for doing something new rather than settling for failure one more time.
It is not easy taking the fight to your own party. But it can, as Cameron has learned, be essential. Such confrontation does not guarantee victory, but avoiding it -- as the former British prime minister's retirement from politics demonstrates -- brings you inevitable defeat.