During the time between World War One and World War Two, coastal defence artillery was designated as 'Heavy Batteries’. The Kent and Sussex ‘Heavy Brigade’ of the Territorial Army became responsible for the defence of the ports of Dover and Newhaven. However, the status of Newhaven had been reduced to Class 'C' - not manned on mobilisation – which meant that when World War II started in September 1939, The 159 Heavy Battery R.A. Territorial Army (most of whom had trained at Newhaven Fort) was mobilised and sent to Dover. There was only a small maintenance squad left at Newhaven Fort, to keep the guns and searchlights in good, serviceable condition.

It was only a matter of days, however, before Newhaven's lower status as a port was reviewed and raised, resulting in part of 159 Battery being brought back from Dover to man Newhaven Fort on much the same strategic principles as were applied in the previous war.