All wars have consequences for domestic politics. The First World War was an important factor in the demise of the Liberal Party. The Second World War made possible a reforming Labour government. The Falklands War helped to bring about a landslide election victory for Margaret Thatcher in 1983.
The war in Iraq has also led to changes that are at least as significant. Tomorrow the Chilcot inquiry questions Tony Blair for a second time about how Britain went to war. The exchanges will be of historic interest but of no contemporary relevance. Inquiries are obsessed with the origins of war. We live now with the consequences.
The most direct change brought about by Iraq relates to the calculations made by a leader of the Labour Party. They are the direct opposite of those made in advance of Iraq. Desperate to purge Labour of its vote-losing past, Mr Blair resolved when he was leader to support the US in more or less all circumstances. After making this understandable but indiscriminately weak-kneed resolution, he was trapped when President Bush moved from Afghanistan to Saddam without reason or much of a pause.
All the overblown speculation about whether or not Mr Blair was a liar is a red herring. He was a political leader caught in an impossible dilemma. Having resolved to support military action in Iraq partly for expedient political reasons, he would have been in the worst of all worlds if he had pulled out at the last moment on the grounds of legality or suspicions that Saddam had no weapons.
Mr Blair strode on partly because there was no room to turn back. He had calculated that it was in Labour's interests, as well as the country's, to stand "shoulder-to-shoulder" with the US and in doing so to retain the enthusiastic support of Rupert Murdoch and his newspapers. I am not arguing that these were the only reasons why Mr Blair supported the war, but like any leader he was bound to make political calculations about where such a venture would leave him and his party.
After Iraq, Ed Miliband makes a very different calculation. In order to break free from the trauma, he assumes that public repudiation is essential. Miliband was accused of framing such a message solely in order to win the leadership contest. This is not the case. He also made his opposition to the war a central point in his first speech as party leader. The message was aimed at the wider electorate disillusioned by Iraq. Any new leader was bound to calculate that it was in his and his party's interest to move on, making it possible to engage once more with voters that had turned away, and perhaps at some point to open dialogue with Liberal Democrats, who had in their opposition to the war a distinctive pitch.
No Labour leader brought up on the politics of the 1980s and seeking election victory would have confidently made such a calculation in advance of Iraq. An adviser who worked with Gordon Brown told me the other day that Mr Brown had doubts about the war, but unlike some of Mr Blair's domestic policy adventures, the Chancellor made no attempt to block the route to war.
Mr Brown's calculations were the same as Mr Blair's – fear of appearing weak on defence and anti-American, and fear of the backlash of Mr Murdoch and others. The adviser, who strongly opposed the war, was shocked at the gap between doubts and political expediency but accepts he was probably naïve.
In the post-Iraq era it would be naïve to argue that electoral victory was dependent partly on a willingness to wage war with the US. Miliband has been explicit in saying that Labour's foreign policy will be determined by values rather than alliances. The practical implications of such a declaration are almost comically vague, but the politics is clear. Labour needs to leap away from some of the more defensive assumptions in relation to the alliance with the US that led Mr Blair towards war.
The leap includes a tentatively less servile approach to Mr Murdoch and his newspapers. They still matter to any Labour leader and I have no doubt that Mr Miliband would die for a positive leader in the Sun or The Times (his life is not in danger), but the fear of breaking loose is not as great as it was before Iraq. Mr Murdoch's newspapers threw their weight behind the Conservatives at the last election and still fizz with positive coverage, but with limited electoral benefit for David Cameron. Iraq was partly the consequence of Labour's fear of some newspapers.
The war has had consequences for Conservative foreign policy too. One of Mr Blair's worries after the election of President Bush was that the Conservatives would be seen as admirably close to the new regime, when as a Labour leader he would be more distant.
The leader at the time, Iain Duncan Smith, was a fervent Atlanticist who was widely praised in most newspapers for declaring his support for American military intervention, whether or not President Bush bothered to seek UN authorisation. On this at least Mr Duncan Smith seemed to be part of the fashionable orthodoxy of the time, especially after September 11.
There is no such orthodoxy now. Mr Cameron is more pragmatic than some of his predecessors and has the political space to be so – more secure in his vocation than Mr Blair, who was haunted by the image of Neil Kinnock being treated dismissively by President Reagan shortly before the 1987 election, and less ideological than Mr Duncan Smith. It is no longer certain that a Conservative administration would support US military action in the future. After Iraq, political calculations do not propel a Conservative or a Labour Prime Minister towards conflict.
The dynamics of the Coalition would be a constraint on Mr Cameron too in the current circumstances. The Liberal Democrats opposed the war and on this issue are still united in their sense of vindication. Until the war the fashion in British politics was to hail ostentatiously strong leaders, those who led from the front without bothering very much with anyone else.
Mr Blair had observed Margaret Thatcher and John Major at close quarters. The more consensual Mr Major was regarded widely with scorn. Mrs Thatcher's "strong" leadership was seen as an asset for her government and party. Mr Blair opted for the Thatcher style, allowing only Gordon Brown and parts of the media to stand in his way.
He had no reverse gear. He had little time for scrutiny by Cabinet. But after the war critics wondered about the level of scrutiny and whether it was healthy for a leader to press on unchecked. The culture of coalition in which internal disagreement and compromise are unavoidable makes Thatcher-style leadership impossible and comes at a point where, after Iraq, the style is conveniently out of fashion. Where voters once hailed "strong" leadership they now welcome parties working together. As George Osborne observed in a recent speech, the Coalition has brought back Cabinet government. Iraq made it possible for the return to be met with approval.
On the domestic front Mr Blair still holds extraordinary sway. His public service reforms, which sometimes had a chaotic quality in their more limited manifestations, are now being fully realised by David Cameron. Senior Tory ministers idolise Mr Blair. But in terms of the calculations that led to the nightmare of Iraq, he is a ghostly figure from another age.