Richard Seymour is a writer better known, through one of those pseudonyms that throw employers off the scent of those blogging in work time, as "Lenin". The gall of the name gives a hint of the sly wit of his blog, Lenin's Tomb, which has since 2003 been the online left's most con sistent, trenchant and mordantly funny source of information and polemic, besides offering excursions into history and political philosophy.

"The Tomb" has been noted for attacks on the "pro-war left", those liberals and ex-socialists associated with various convocations - the blog Harry's Place, the Euston Manifesto - which argue that the "Islamofascist" enemy must be fought by any means necessary. Essentially, this is the subject of Seymour's first book.

The Liberal Defence of Murder would have been enjoyable enough as a cut-and-paste collection of tidied-up Tomb posts, but over and above this it is a freshly written, heavily footnoted and clearly obsessively researched history of 400 years of the "decent left", from the Glorious Revolution of 1688 to Operation Iraqi Freedom in 2003. This leads to minor flaws. The transfer from Seymour's pugilistic and waspish blogging manner to a more sober historical account leads to a slight loss of stylistic panache. The moralistic title is also unfortunate. Nonetheless, these are small gripes - Seymour overwhelmingly manages the transition from blogger to historian with great assurance.

Although by its own account the pro-war left emerged as a reaction to 11 September 2001, thereby "changing everything", Seymour restores a welcome sense of historical perspective to a discussion that too often shelters inside the shrill bubble of the media. The overarching concern of this book is with the intellectual apologias for capitalist expansion and empire that have existed since the very inception of capitalism, whereby the barbarities of imperialism are justified with recourse to the need for civilising the barbarians - the recent pro-war left being seen as an extension of this, not as a new development.

The first chapter is concerned with how certain Enlightenment thinkers refused to extend their project to "savages", and so we begin with the theoretical subordination of humanity to imperial property by Grotius and Locke. The concentration on ideas and thinkers is never empiricist anti-intellectualism: Seymour explains that others, such as Immanuel Kant, were far less convinced of imperialism's murderously civilising influence. The vicious imperialism of the 19th century is found to be the heyday of an enthusiastic, if rhetorically tortuous intellectual imperialism, exhibited by "humanitarians" such as John Stuart Mill and Alexis de Tocqueville. The account makes clear the fit between rhetoric and a reality of starvation and exploitation. The democratic ideal, it argues, curdled into "Herrenvolk (master race) democracy" solely for the conquerors, simultaneously pioneering a pseudo-scientific racial theory.

Even socialism, particularly in its more gradualist, reformist version, was not immune to an affection for the imperial bloodbath. The Fabians and the Labour Party (with noble exceptions such as Keir Hardie) were very seldom opponents of empire or of war. This combination of "pragmatism" and acquiescence or participation in the most senseless slaughter reached, in Seymour's account, a bloody apotheosis in the First World War, enthusiastically supported by the German Social Democrats. By contrast, the early Communist parties were unique in their principled opposition to the Great Game, at least until the sordid Comintern politicking of the 1930s. Through these analyses, Seymour makes clear that, in keeping with George Orwell's 1939 essay, the rhetoric of European liberalism always carried the unspoken clause "Not Counting Niggers".

The chapter "Creating an Imperial Constitu ency" notes how the US consciously inherited the imperial mantle in the name of "progressivism". After 1945 a body of reliable liberal opinion willingly excused such events as the anti-democratic coups in Iran and Guatemala, via invocations of liberalism, human rights and the natives' alleged inability to govern themselves. Yet another chapter involves an intriguing discussion of the much-mythologised neoconservatives. Charting the derivation of some neocons from an American post-Trotskyist milieu, Seymour makes clear that the notion of the neocons as a sinister intellectual cabal is an anti-intellectual overstatement. Rather, they were just the most recent and vociferous of generations of allegedly rationalist enthusiasts for blood and soil, from Heidegger to Leo Strauss.

There is a concluding analysis of the recent pro-war left, from those preposterous ex-Maoists, the nouveaux philosophes, to Paul Berman (whom we first meet writing in defence of the CIA's Nicaraguan death squads), Michael Ignatieff and Christopher Hitchens, cheerleading wars in Yugoslavia, Afghanistan and, above all, the gro tesque catastrophe in Iraq. Nonetheless, The Liberal Defence of Murder is probably more valuable as history than as polemic. It delves into areas that are usually politely ignored, carefully uncovering liberalism and reformism's own shameful record of collaboration with mass murder. Besides, if (as seems likely) the new US Democratic administration rehabilitates "humanitarian" imperialism, this book will be essential reading as a reminder of previous form.