"Mr Blair, this is a citizen's arrest for a crime against peace, namely your decision to launch an unprovoked war against Iraq. I am inviting you to accompany me to a police station to answer the charge." If you happen to encounter Tony Blair, this is what the multipurpose-campaigner and Guardian columnist George Monbiot encourages you to say by way of introduction. Indeed Monbiot is so keen on this form of address that he has set up a site — ArrestBlair.org — which offers a bounty raised by public subscription, standing at £2,147.54 at the time of writing, to anyone who attempts to use this salutation on Blair. Monbiot has form here, having attempted to "arrest" the Standpoint contributor and former American ambassador to the United Nations and Under-Secretary of State John Bolton for alleged war crimes at the Hay-on-Wye book festival. Helpfully for Bolton, Monbiot announced his plans in advance and was removed from the premises by security. 

Opposition to Blair and the Iraq war is only one narrow front of 50-year-old Monbiot's lifelong campaigning. His first big cause was the fate of native peoples and environmental degradation in the Third World, with angry polemics-cum-travelogues about Brazil, West Papua and East Africa. After this, and turning his emphasis on Britain, he became the media voice of the protest movements of the early 2000s-      anti-corporation, anti-globalisation, anti-roads. As for so many of the protestariat, climate change and what to do about it became the big issue and often the totem pole on which to hang pre-existing demands.

Monbiot's current obsession and the subject of his recent book (Feral, Allen Lane, £20) is "rewilding". He argues that because of over-farming much of the world, and Britain in particular, has become a degraded and diminished place. Nature should be allowed to take its course, moorlands should gradually become forests again, and Britain's uplands should be cleared of the "white plague", otherwise known as sheep. 

Species which once roamed Britain's wilds should be reintroduced —starting with wolves and beavers, previously last found in the 17th and 18th century. Why should elephants (last seen in the UK 115,000 years ago) and rhinos not roam our lands? Monbiot writes, "I have seen no discussion about the reintroduction of elephants to Europe, though I would like to start one."  

There is a problem for Monbiot with his enthusiasm for rewilding — he needs to separate his "good" rewilding from the nasty sort. As Monbiot acknowledges, some of the places where forests and wildlife are now booming and which were once overfarmed, degraded landscapes have "recovered" thanks to human catastrophe — areas such as south-western Slovenia where the population collapsed due to the horrors of the Second World War and subsequent ethnic cleansing. But there is an even greater problem.  Many of those who have been the most enthusiastic advocates of rewilding are precisely the people Monbiot most despises — rich landowners.  

The common thread which runs through Monbiot's campaigns and writings is a hatred of what he perceives as the concentrations of wealth, power and privilege. Monbiot says of himself, "I was born into the third tier of the dominant class," and sees his role as battling against the interests of that class. His father, Raymond Monbiot CBE — a scion of the Salmon family, which owned J. Lyons and its Corner Houses — has been a deputy chairman of the Conservative party; George's mother, Rosalie Monbiot CBE, herself the daughter of a Conservative MP, was a Tory councillor for 31 years.

George was sent to Stowe, a relatively recently established boarding school — it is celebrating its 90th birthday this year — but one which is housed in perhaps the most splendid buildings and grounds of any public school, certainly of any school of recent vintage. It occupies a magnificent Palladian mansion near Buckingham built for the first Viscount Cobham. Its gardens were designed by Capability Brown —Monbiot says that in his time "the gardens (a landscaped deerpark) were a vast playground of crumbling follies and overgrown lakes, of coverts and laurel brakes in which ruined monuments could, like Mayan temples, be discovered by adventurous boys".  

Writing in 2004 at the height of the hunting debate, Monbiot quotes a letter to the school magazine from one of the organisers of the Countryside Alliance, the campaign opposing the ban on hunting: "There were five people present in our operations room at Kennington Road early one morning — I realised that we were ALL Old Stoics! . . . I developed my feeling for the countryside at Stowe."  

Monbiot then writes: "I too developed my feeling for the countryside at Stowe and for the kind of people who run it . . . As an animal welfare issue, fox hunting comes in at about 155 . . . as a class issue, it ranks behind private schooling at number two . . . This class war began with the Norman conquest. It still needs to be fought." It is here that George's politics were formed — he is still fighting battles against the boys who shared his dorm.