Why British forces were already watching for minefields

Reconnaissance flights, passing ships and submarines watching the coast all offered opportunities to detect the preparations. The proposed minefields occupied about four separate zones, making it harder to imagine every observation missing them. British forces already watched for German minelaying as part of ordinary wartime operations. In the First World War comparison, Germany devoted a smaller proportion of its effort to submarines yet came closer to starving Britain than it did in the Second World War. The Royal Navy attributed that danger to submarine minelaying. On this reading, Britain's earlier vulnerability helps explain its concentration on mining and countermining by 1940.