In 1917, Britain was in danger of defeat. Germany’s U-boats (submarines) were choking seaborne commerce. In April 1917 alone, 169 British merchant ships were sunk, as were a quarter of all merchant ships sailing from British ports that month, and outrage at Germany’s unrestricted submarine warfare caused the United States to enter what was then called the Great War. But it would be months before the U.S. Army and Marine Corps could be in action in France and Belgium. Britain had to overcome the U-boats.
This was a slow and costly battle, using merchant ship convoys, more capable aircraft and warships – British and American – and new technology weapons (such as the depth charge) and sensors (the hydrophone, the original passive sonar). The most potentially decisive anti U-boat approach was “attack at source”: striking at the U-boat bases, which were heavily defended with minefields, coast artillery batteries, antiaircraft guns, and reinforced concrete submarine pens.
The main U-boat bases in Germany were beyond striking range. Those at Zeebrugge and Ostend in German-occupied Belgium appeared to be potential targets. U-boats, shuttling between the ports using Belgium’s inland canals, penetrated British minefields that protected the cross-Channel lines of communication 30 times a month. German destroyers and torpedo boats based there were in position to raid British shipping.
These bases were to have been the target of a British infantry division making an amphibious landing on the Belgian coast, in support of the 1917 summer offensive. But the British, reluctant to carry out a major amphibious operation after being defeated by the Turks at Gallipoli in 1915, cancelled the operation. Instead, Royal Navy monitors bombarded the two bases. British aircraft attacked them. The U.S. Navy, a supporter of “attack at source,” organized a Marine Corps air wing to join these attacks, but it would take months to become operational.
Planning the Raid
In January 1918, Royal Navy Vice Adm. Roger Keyes took command at Dover. An aggressive commander, even though he lacked familiarity with special operations or the Belgian coast, his plan was to “strike at the root of the evil by attempting to block the sea-exits” at the two bases, sinking obsolete concrete-filled warships with skeleton crews and a few guns – blockships – in the channels and sealing the U-boats and destroyers in harbor. To prevent German coast defenses from sinking the blockships as they approached at Zeebrugge, the modified obsolete cruiser HMS Vindictive would carry the 4th Battalion Royal Marines and sailors to launch an amphibious assault against coast artillery positions on the mole, a mile-long projecting sea wall around the entrance to the harbor and canal, connected to the mainland by a viaduct, which would have to be destroyed to prevent German reinforcements reaching the battle. Keyes’ plan relied heavily on extensive smokescreens to block German coastal guns from sinking either the blockships or the mole assault force.
Keyes wrote that he “was given an absolutely free hand, not only to make my own plans for the blocking of Zeebrugge and Ostend, but to select all the people who carried them out.” Keyes’ complex plans were hastily drawn up and had many potential points of operational failure. Intelligence to support the planning was limited, in part because of the need for strict operational security.
The Royal Navy of 1918 had no preexisting special operations forces. Both the amphibious assault force and ships’ crews were volunteers. Selected fleet-wide for an undisclosed hazardous mission, they included Canadians, Australians, and New Zealanders. Each part of the raiding force trained and rehearsed separately, starting in January 1918.
Keyes planned two complex, simultaneous direct attacks involving some 150 vessels, including escorts. Two heavily armored monitors with 15-inch guns, HMS Erebus and Terror, would provide naval gunfire support. Coastal motorboats – predecessors of the World War II motor torpedo boats – and motor launches would attack defenses, rescue blockship crews, and put down smokescreens. The force was ready at the end of March. British forces were hard pressed on the battlefields of the Western Front by the German spring offensives. There was a great need for a dramatic operation to boost morale.
On April 11, wind, weather, moon, and tide conditions all favorable, the force set sail, under conditions of wireless silence, for the Belgian coast. Strong fighter escort prevented German air reconnaissance. Keyes was in command, aboard the destroyer HMS Warwick. When the wind shifted unexpectedly, making the all-important smokescreens impossible, Keyes felt he had no choice but to send a recall signal.