Wu’s iron chains and river spikes

Wu’s iron chains and river spikes were a river defense devised by the eunuch Cen Hun during the Jin invasion of Wu. Intended to halt the attacking navy, the barrier instead became an obstacle the Jin fleet could overcome: Wang Jun sent fire ships against it, melting the chains and breaking apart the spikes so his ships could continue downstream.

Construction and Purpose

As Sima Yan’s forces prepared to attack Wu from several directions, Sun Hao was alarmed by the prospect of a naval assault. Cen Hun recognized that the river attack posed a particular danger and proposed a physical barrier: iron gathered from across the land was cast into thousands of heavy chains and large spikes. The chains were stretched across the river, while the spikes were placed in the channel to damage or sink incoming ships.

The design addressed the problem of blocking a fleet by turning the river itself into a defensive line. Its scale depended on extensive material collection and on positioning the chains and spikes across the waterway. Yet its effectiveness rested on the assumption that the Jin ships could not overcome the metal barrier.

The Jin Countermeasure

Spies informed Admiral Wang Jun of both parts of the defense: the chains stretched across the river and the spikes sunk beneath the surface. Rather than attempt to force the fleet through them, Wang ordered large fire ships prepared. His sailors sent the vessels toward the barrier and set them alight shortly before they reached it.

The resulting heat melted the iron chains and broke apart the spikes. With both elements of the obstacle disabled, Wang Jun’s navy sailed on. The defensive works did not merely fail to stop the fleet; their materials and placement made them vulnerable to a countermeasure that cleared the route for the invasion.

Strategic Consequences

The barrier’s destruction formed part of a broader collapse of Wu’s defenses. Jin forces were advancing on multiple fronts, while earlier defeats and the loss of cities weakened Wu’s position. Once Wang Jun’s fleet passed the river defenses, the Jin advance continued toward Jianye, the Wu capital. Wu commanders debated surrender, but Zhang Ti argued that the state should not fall without a fight; the ensuing battle ended with the deaths of Zhang Ti and Shen Rong and the disintegration of the Wu army.

The episode presents the chains and spikes as a conspicuous, large-scale answer to an immediate military threat, but one that could not withstand an opponent who learned of the plan and adapted. Their failure helps turn a defensive measure into a marker of the invasion’s momentum: once the river barrier is gone, the navy can continue its advance.