Han in 2002 extended this from attacks on vertices to attacks on edges; here, the attacker removes edges connecting high-degree nodes, and again, past some critical point, the network becomes disconnected [@Holme02]. They also suggested using [*centrality*]{} – technically, this is the ‘betweenness centrality’ of Freeman [@F77] – as an alternative to degree for attack targeting. (A node’s centrality is, roughly speaking, the proportion of paths on which it lies.) Computing centrality is harder work for the attacker than observing vertex degree, but it enables him to attack networks (such as beta networks) where there is little or no variability in vertex order. Finally, in 2004, Zhao, Park and Lai modelled the circumstances in which a scale-free network can suffer cascading breakdown from the successive failure of high-connectivity nodes [@ZPL04]. These ideas find some resonance in the field of strategic studies: for example, Soviet doctrine called for destroying a third of the enemy’s network, jamming a further third, and hoping that the remaining third would collapse under the increased weight of traffic.

Naive Defences Don’t Work
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Given the obvious importance of the subject, and the fact that the Albert-Jeong-Barabási paper appeared in 2000, one obvious question is why there 