We model the decentralised defence choice of agents connected in a directed graph
and exposed to an external threat. The network allows players to receive goods
from one or more producers through directed paths. Each agent is endowed with
a finite and divisible defence resource that can be allocated to their own security or
to that of their peers. The external threat is represented by either a random attack
on one of the nodes or by an intelligent attacker who aims to maximise the flow disruption by seeking to destroy one node. We show that under certain conditions
a decentralised defence allocation is efficient when we assume the attacker to be
strategic: a centralised allocation of defence resources which minimises the flow disruption coincides with a decentralised equilibrium allocation. On the other
hand, when we assume a random attack, the decentralised allocation is likely to
diverge from the central planner’s allocation.