In 1996, when you travelled through the mountains of Chiapas, Mexico, soldiers would get on the buses every half an hour or so to check on the passengers. They'd look at papers, search random bags and occasionally take people off for a chat. Some did not get back on.

There was an ongoing guerra sucia in Chiapas. Two years earlier, when the Zapatista armies had risen up out of the jungle, there had been hundreds of casualties in the ensuing conflict. In the seven months I was there, several Zapatistas were killed in skirmishes. Then came the massacre of Acteal, where 45 Zapatista supporters, mostly women and children, were murdered by unknown paramilitaries. The mood was oppressive, skittery, explosive.

Many activists in this country are supporters of the Zapatistas; they were an inspiration in the 1990s. But even the most partisan of fans can understand why the Mexican government saw them as a serious threat. They had trained an entire army in secret; they had called for revolution; they maintained an autonomous zone and moved around in that area with their guns and ammunition belts in clear view across their chests.

There is a huge difference between the Zapatistas and our climate activists - you only need a drop of common sense to see this. Yet recently, border officials in the UK used the Terrorism Act to get four environmental campaigners off coaches for questioning, in a manner that reminded me strongly of Mexico. Chris Kitchen was one of the four, on their way to Copenhagen for a pre-conference planning meeting. He was questioned for 30 minutes about his work, his education and his family. The coach left without him and he was finally allowed to travel the next day.

Can these climate activists seriously be suspected of terrorism? How have we come so far from any kind of sense about what constitutes a genuine and serious threat to national security? Much has been written about the vast amount of terrorism legislation that Labour has introduced, but you may not know that Blair's government deliberately rewrote the definition of terrorism so that animal rights and environmental activists could be included: "Acts of serious violence against people and property have undoubtedly been committed in the UK by these domestic groups," said a Home Office consultation paper on the new law. "There is also the possibility that new groups espousing different causes will be set up and adopt violent methods to impose their will on the rest of society."

OK, we can get our heads round that. But does anyone seriously believe that Climate Camp or Climate Justice Action - the group that Chris Kitchen was going out to meet - plan to use violent methods? This particular bunch of activists are wedded to non-violence: the idea that they would plan to hurt anyone deliberately is nonsensical. These people have disabled toilets and fire exits at their camps, for God's sake.

The problem is that the terrorism legislation has been written so incredibly broadly that it is dependent on police and the courts using a lot of common sense. I have an uneasy feeling that there is less and less of that around.

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