After last night's vote, how will Mr Blair be judged by history?

It's impossible to be sure that we doubters will be proved right, but the evidence is not yet there to justify war

The historical parallels, all of them deeply imperfect, just keep piling up. Last night's revolt against government strategy on Iraq was much larger than expected, painfully underlining the crisis Tony Blair could face if he fails to secure a second UN resolution. But for many older Labour MPs, who trooped through the anti-government lobby in by far the biggest revolt since Tony Blair came to power, the most resonant folk memory is the story of another war and another Labour prime minister caught between the conflicting demands of his own party and the transatlantic alliance: Harold Wilson and Vietnam. A story in which, for some, Wilson appears in the unlikely role of hero.

This is rich in ironies. By the late Sixties, the Wilson government was consistently vilified from the left for his public support for the US in Vietnam. Indeed, that support did much to alienate, irrevocably, the idealistic (and not so idealistic) left which had thrilled to Wilson's victory in 1964 after 13 years of the Tories. It was largely because of Vietnam that, as Ben Pimlott, Wilson's biographer, put it, "intellectual fashion, most powerful of political motivators, moved away, and never returned".

The reason for today's revisionist reading in the shadow of Iraq, however, is that Wilson's strategy, maintained throughout his premiership, was, in response to continued pressure from Washington, (again in Pimlott's words) "to give the Americans everything they wanted, short of what they wanted most, which was British troops in Vietnam". This was by no means as easy as his critics on the left presumed. From his first visit to Washington in December 1964, Wilson faced demands from Lyndon Johnson to commit British forces.

He resisted them despite a painful economic dependence on the US. And he did so although, like Tony Blair today, he faced a (rather more potent) opposition willing to back the US, right or wrong. Sir Edward Heath is now the venerable doyen of peaceniks. He wasn't when he was Leader of the Opposition. He repeatedly criticised Wilson for being too lukewarm in his support for the Americans. It's likely that in government he would have committed British troops. And when Wilson finally drew his line in the sand and condemned the US bombing of Hanoi and Haiphong, Heath justified the bombing on the ground that the war would not end until the Vietcong realised they could never win.

Wilson's stance wasn't particularly glorious. Some of his repeated attempts at peace-making ­ like his ill-starred dispatch of the MP Harold Davies to Hanoi ­ were embarrassingly half-baked. But in retrospect he was right where Heath had been wrong. The domino theory of Communism that had sustained the Korean war was no longer universally applicable, particularly at a time when détente was just beginning. Vietnam was a tragedy, not least for the US, an unwinnable war. Whether for reasons of mere party management, or for higher motives, Wilson now looks in hindsight to have been pretty far-sighted to keep British forces out of it.

Now, history does not repeat itself. The "parallel" shouldn't be strained. An Iraqi war, for a start, is winnable, perhaps quite swiftly, though at what cost in civilian and military life can't of course be predicted. Secondly, the opposition too is rather different. The demonstrations against it have been much, much larger, much more peaceful, and much earlier in the crisis than those against the Vietnam war, which didn't peak until about 1968. That's something for the Government to worry about. But another difference in one sense works oddly in favour of the Government's sometime flirtation with regime change as an objective, however legally and politically dangerous that objective is. Asked whether we would have regarded Ho Chi Minh as a preferable leader of a unified Vietnam to all the realistically available alternatives, most of us who demonstrated against that war, frankly, would have said "yes". No one on that huge demonstration 11 days ago, from the SWP to the Women's Institute, would say that for a second about Saddam Hussein. Even those most opposed to war yearn to see him go.

What the Wilson story does show, however, is that the rightness of a decision on war will take much longer to judge than a few intense weeks. Conventional wisdom has it that Mr Blair is facing his most perilous period; in the absence of a UN resolution his own leadership could be at risk, particularly if the war is bloodier and longer than predicted. But conventional wisdom also has it that a swift, decisive victory, underpinned by relief and celebration by Iraqis, would have an entirely opposite effect. You don't have to believe, in the cynical words of one (pro-war) frontbench Tory yesterday, that he will personally "go to Baghdad in triumph in time for the local elections" to see that his premiership will be immediately strengthened.

But the big variables are far too unpredictable for that to be the end of the story. Many, though not all of them, were identified in the long series of backbench speeches yesterday in favour of Chris Smith's delaying amendment, some of the most persuasive by the Tories Kenneth Clarke, Douglas Hogg and John Gummer. Won't a pre-emptive invasion now increase rather than decrease the prospects of more terrorist massacres in the West? Isn't the difference in the treatment of North Korea and Iraq an invitation to non-nuclear states to become nuclear? Where is the concrete evidence that an allied victory in Iraq would give the long-awaited push to a just peace in Israel-Palestine which Mr Blair has to his credit urged on George Bush? What are the long-term plans for stabilising and democratising the country after war? Of course, it's impossible ­ even unreasonable ­ to be sure that we doubters will be proved right. Or not to be moved by the almost unbearable catalogue of atrocities by the Baghdad regime unveiled in a powerful pro-war speech by Ann Clwyd, the Labour MP.

But as Mr Clarke pointed out ­ while praising Mr Blair's decision to engage with the US presidency and push it towards the UN ­ it still looks, not least to Middle England, as if this is a preordained war to a US timetable, and the evidence is not yet there to justify it.

If the UN resolution is finally secured, the war will probably have to be supported, however reluctantly, because anything else will reinforce an even more dangerous US unilateralism, shatter the authority of the UN and reduce the chances that the post-war future of Iraq will be in UN hands, as it should be. A noticeable, if subtle, shifting of the parameters yesterday in which Mr Blair would be prepared to take action, indicates that ministers are far from confident that such a resolution can be achieved. At the same time, last night's vote could be an ominous harbinger of how the Commons would divide if the resolution is not secured. That makes life even more difficult for Mr Blair. But, as yesterday's debate graphically underlined, the risks have a global reach which extends way beyond the fate of the British premiership.

d.macintyre@independent.co.uk