Last night in the Commons a great switch was thrown in the national psyche and nothing may ever be quite the same again. This is not a left-right shift, but a long-delayed acceptance that Britain is less powerful and poorer than it was, weary of wars and no longer proud to punch above its weight. No more pretending, no more posturing.

Next week Rule Britannia will belt out loud as ever at the Proms in that partly ironic parade of cheerful patriotism. But the great game is over. Poor David Cameron has been the one left stranded when the music stopped, still singing as everyone else falls silent. From Number 10 came effing and blinding at Ed Miliband, calling him, as reported in the Times, a "f****** c*** and a copper-bottomed s***". But it wasn't Labour, it was Cameron's whole country who had changed while he wasn't looking. Cue last-minute key change in Downing Street's unconditional promise to the US, but he's still out of tune with a country that doesn't want to go to war.

As the true meaning of end of empire sinks in, great questions follow. Why continue to spend more than comparable countries on defence? Why do we (and France) still squat in UN security council seats? What is the point of Trident, dependent entirely on the closest allegiance to America?

Consider the breadth of opposition to a Syrian intervention. First come the British people, two to one against war, with YouGov finding opinion hardening. Outside the gates of Downing Street stand Stop the War demonstrators. Biting at the heels of anxious Tory MPs comes Ukip's Nigel Farage accusing Cameron of "his greatest misjudgment yet" and gloating that his anti-war stance is "the single most popular thing Ukip's ever said". Now add in warnings from the assemblage of military top brass (retd).

Inch nearer to the core of conservatism and yomping, Sir Max Hastings is against, as is our own Sir Simon Jenkins, always for the state doing less. The Mail opposes: "If MPs have the slightest suspicion that attacking Syria will cause more suffering than it can prevent they have a fundamental duty to vote 'No'." They roll out the erstwhile warhorses, with Simon Heffer asking, "How does this relate to Britain's interest?" and Stephen Glover, "We should not trot along obediently as the ever-loyal poodle … We were a global power for 250 years but that period is past … Let's concentrate on our national self-interest." The Express takes the same line, as does the Telegraph, where Peter Oborne calls this "a catalyst and a deadly error" while praising Stop the War for showing "far more mature judgment on these great issues of war and peace than Downing Street, the White House or the CIA". Only Murdoch's Sun and Times hold the Cameron line, minding their owner's US interests. In the FT, Janan Ganesh, George Osborne biographer and confidant, opposes an attack, in line with his forecasts of what a future triumphant small-state Tory party would do, ruling out the expense of military adventures.

Labour's determination to make strict conditions on legality and objectives is what you would expect under a leader whose first act was to repudiate Iraq. Behind him sit benches of MPs bitterly regretting their vote for that war. Let Dame Tessa Jowell, one-time passionate Blairite, stand as bellwether for that sore lesson: she has been re-reading her verbatim notes of the cast-iron but false assurances ministers were given back in 2003. For many who were there, as for Miliband who was not, this is the chance to show that solemn lessons have been learned. Contrary to charges of cheap opportunism, this hasn't been easy for Miliband, wrenching his close links with Obama's Democrats. Labour's left always had an anti-American reflex, but not Ed Miliband's entourage.

Cameron should be effing and blinding instead at his own party, for it's they who have changed without him noticing. With more than 70 MPs threatening to rebel, he lost control while his top ministers, Theresa May and Philip Hammond, allowed their disquiet to be reported, delivering an "et tu, Brute" stab in his front. The new doctrine of revenge as a legal pretext for war alarms many. The party of Suez no longer, the same impulse that makes them anti-EU has them prize British self-interest more than global status.

The horror of the gas attacks is unbearably raw for all to see on YouTube – but the litany of possible terrors that might be unleashed in a regional conflagration were well-rehearsed in the Commons debate. Weighing risks is as difficult as weighing degrees of moral outrage: 1,000 unarmed demonstrators have been killed by Egypt's unelected military government, no one has said intervene. How is outrage rationed? Britain may be coming to its senses about its real status, but there is nothing appealing about the new little England isolationism on the right: let the world go hang, so long as we're alright. It comes with an unpleasant undertow: why fight in Muslim countries for the rights of a lot of Muslims anyway?

Tony Blair's Chicago speech in 1999 calling for intervention in Kosovo gave a briefly inspiring vision of liberal interventionism, with a moral duty to do what you can, where you can to help others. But between that vision and the reality falls the long dark shadow of 12 years of war, in countries that don't thank us, doing more harm than good, killing many and sending home hundreds of our soldiers in coffins. If the US goes ahead without us, just possibly this week the last illusions of empire will be finally laid to rest.