The last German attack on the British anti-submarine barrage across the Dover Straits came on the night of 14-15 February 1918. The previous one on 20-21 April 1917 had resulted in the loss of two German torpedo boats and a clear British victory. The Germans then changed strategy, launching a number of attacks on shipping between the Netherlands and the UK, the largest of which took place on 10 May.
The Germans then sat on the defensive because they feared that the British Passchendaele offensive might include an amphibious assault. The British planned such an operation, but the land offensive did not go well enough for it to be carried out. By the end of 1917 too many vessels had been transferred away from Flanders, mainly to take part in Operation Albion, an amphibious assault in the Baltic Sea, for them to carry out offensive operations.
The attack in mid February was carried out by Korvettenkapitän Heinecke’s 2nd Flotilla of the High Seas Fleet, which sailed from Heligoland Bight without stopping in Flanders in order to achieved surprise. Its eight torpedo boats were meant to sail on 13 February but were delayed a day by bad weather and then reduced to seven when one of them suffered condenser problems.
Heinecke’s plan was to divide his force into two groups. He would lead one, which would attack patrols south of Dungeness and then the patrol line from Folkestone to the Varne Bank. The other, led by Kapitänleutnant Kolbe, would attack the patrols on the south side of the Channel.
Weather conditions on the night of 14-15 February were ideal for a raid. The weather was fine but overcast, the sea was unusually calm for the time of year and visibility was variable, with patches of haze. The minefield was illuminated by flares and searchlights in order to detect surfaced U-boats, but this blinded the vessels burning them, produced smoke and potentially obscured warning lights and gun flashes.
The British had a light cruiser and three destroyers in the Downs, two destroyers on the West Barrage Patrol, four destroyers on the East Barrage Patrol and two paddle minesweepers, a monitor, a destroyer, a Patrol boat, two French torpedo boats and 10 trawlers supporting the 58 drifters patrolling the deep minefield. There should have been a monitor with 12 or 15 inch guns on duty but none was available so the 7.5 inch gunned HMS M26 was on duty.
Between 11:30 and midnight on 14 February the drifter Shipmates (Lieutenant W. Denson RNR) spotted a submarine and fired the appropriate warning signal of red and white lights. The submarine soon disappeared. Around 00:30 am two German destroyers fired on the paddle minesweeper HMS Newbury. She was set alight and was unable to fire the green warning signal for surface raiders. British ships that heard the gunfire assumed that it was British ships attacking the U-boat.
Denson of the Shipmates saw the gun flashes and realised that a German destroyer attack was underway. Before he could report it, his drifter was caught in German searchlights and his division was under fire. Presumably fearing that his vessel might be captured, he threw his confidential books overboard. The Shipmates managed to escape but did not fire a warning signal as Denson had seen two or three signal rockets. He could not send a coded radio message as he no longer had any codebooks and he had been ordered not to send uncoded messages.
The British failed to realise what was happening, assuming that gunfire was aimed at the U-boat spotted by the Shipmates and that any destroyers that they spotted were friendly. Even the captain of a British motor launch fired at by German destroyers assumed that they were British and had mistaken his vessel for a U-boat. The Germans sank seven drifters and a trawler and severely damaged five drifters, a trawler and a paddle minesweeper; 89 British officers and men were dead or missing. Vice Admiral Sir Roger Keyes, C.-in-C. Dover Command, was not certain of what had happened until nearly 3:00 am, by when the Germans were on the way home.
The British held a Court of Inquiry, which highlighted the faults in the processes for challenging, reporting and signalling in the Dover Straits. Lieutenant Adam Ferguson, captain of the destroyer HMS Amazon, challenged an unknown ship three times, which he said was the normal procedure. He was then supposed to open fire but the ship was by then out of sight. Ferguson was Court-Martialed and severely reprimanded. He and his immediate superior, Commander Bernard of the destroyer HMS Termagant, were both relieved of their commands.
Keyes subsequently issued new orders stating that ‘Suspicious vessels are to be regarded as enemy, unnecessary challenges are to be avoided.’ Offensive action should be taken against ships that did not immediately reply to challenges.
This was the seventh and last German attack on the Dover Barrage. Six of them had been successful with only the raid of 20-21 April being a British victory. However, the attacks came at least a month and as much as nine months apart, with the result that the losses from one raid had always been replaced by the time of the next one. The Dover Straits Barrage therefore continued to keep U-boats out of the busy shipping lane of the English Channel, and to force them to sail round the British Isles on their way to the Atlantic, reducing their time on station. It is unclear why the Germans stopped attacking the Dover Barrage, especially when their last effort was so successful.
 M. D. Karau, The Naval Flank of the Western Front: The German Marinekorps Flandern, 1914-1918, pp. 161-65.
 Ibid., p. 176.
 Naval Staff Monograph (Historical) 1922 vol. vi, The Dover Patrol i. p. 104.
 J. S. Corbett, H. Newbolt, Naval Operations, 5 vols. vol. v, pp. 210-11.
 Ibid., p. 212.
 Ibid., pp. 212-13.
 Ibid., p. 217.
 Ibid., p. 216; A. J. Marder, From the Dreadnought to Scapa Flow; the Royal Navy in the Fisher Era, 1904-1919, 5 vols. vol. v, p. 44.
 Marder, From, p. 45.
 Corbett, Newbolt, Naval, p. 217; Karau, Naval, p. 179.