The island of Schouwen-Duiveland lay west of Tholen and north of the islands of Noord-Beveland and Walcheren. Separating these islands was the broad expanse of the Oosterschelde. From a strategic perspective, Schouwen-Duiveland held little significance for either the Germans or the Allies. Nevertheless, as part of the province of Zeeland and home to a small military presence, it remained a potential theatre of operations.
The island's military value rested largely on two assets: Haamstede Air Base and a coastal artillery battery.
Schouwen-Duiveland had witnessed little military activity during the preceding days, apart from repeated Luftwaffe attacks on Haamstede Air Base. On the evening of 14 May, however, three French naval vessels - Chasseurs no. 6, 9, and 41 - arrived and moored in the small harbour of Zijpe, near Bruinisse in the island's northeastern corner. Earlier that day, the vessels had conducted reconnaissance patrols in the Oosterschelde.
These patrols were most likely prompted by General Durand's persistent concern that German forces might attempt an amphibious crossing from Tholen to Walcheren. During the night, the French vessels put to sea once more to conduct additional sweeps of the Oosterschelde. As previously described, they would also provide artillery support during the German assault on the Zanddijk Line.
Meanwhile, the remaining flying personnel and ground crews of Haamstede Air Base - approximately ninety men in total - embarked at the harbour of Zierikzee and left the island. They disembarked in Zeeuws-Vlaanderen, the southernmost part of Zeeland, from where they continued their journey to Boulogne-sur-Mer in France. Following the collapse of France, many would eventually reach England and go on to serve in the Dutch squadrons of the Royal Air Force.
The modest military force remaining on Schouwen-Duiveland received orders to occupy the island's southwestern coastline and defend it, as far as possible, against any attempted German landing. The defenders possessed virtually no artillery. Their only significant weapons were three 40 mm anti-aircraft guns - which could also be employed against surface targets - and the fixed coastal battery. Among the troops were approximately 150 men who had previously withdrawn from the Peel-Raam Line and reached the island via Tholen. The few heavy machine guns available were positioned to cover the waters of the Oosterschelde.
That evening, the distant rumble of artillery and explosions from the fighting on Tholen could clearly be heard across the water. In response, the local commander dispatched several vessels to Tholen carrying demolition parties. Their mission was to destroy any shipping that might otherwise fall into German hands.
German troops were also reported on St. Philipsland. These were patrols belonging to Gruppe Wirtz. St. Philipsland was a small peninsula north of Tholen, separated from Schouwen-Duiveland by the narrow waterway known as the 'Zijpe', barely a nautical mile wide. Owing to its limited strategic importance, the peninsula had not been occupied by Dutch forces.
In the early evening, the Dutch minelayer Hr.Ms. Hydra, which was guarding the Zijpe, opened fire with her three 75 mm guns on confirmed German patrols observed on St. Philipsland. The narrowness of the waterway, however, allowed the Germans to respond effectively with 37 mm anti-tank guns. Before long, several rounds struck the vessel. The boilers were hit, and serious leaks below the waterline caused water to pour into the ship.
Recognising the danger, the commander decided to beach the vessel beyond the effective range of the German guns. At approximately 2100 hours, the crew deliberately ran Hydra aground on a nearby sandbank.
Up to that point, the engagement had caused no fatalities. Tragically, however, as crew members evacuated the stranded vessel in small boats, nervous Dutch coastal guards mistook them for enemy troops and opened fire. Before the misunderstanding could be resolved, a senior non-commissioned officer had been killed by friendly fire.
The story of Hr.Ms. Hydra did not end there. The vessel was later salvaged by the Germans and eventually broken up for scrap. In 1940, Germany's war industry had an insatiable demand for steel, and even a damaged minelayer represented a valuable source of raw material.