USSR-German collaboration. As part of the Molotov-Ribbontrop pact extension following the partition of Poland, USSR offers Germany a northern base ‘Basis Nord’ to support their blockade of Britain. Naval High Command sends U-36 and U-38 to scout the proposed location at Zapadnaya Litsa on the Kola Peninsula, 25 miles from Mumansk.
Anglo-French Collaboration. Supreme War Council meets in Paris. They agree to an immediate advance to the River Dyle between Antwerp Line and Brussels if the Germans invade (the Dyle Plan or "Plan D"). However, the French turned down proposals to bomb industrial targets in the Ruhr fearing Luftwaffe retaliation against Britain and France.
German destroyers, Z11 Berndt von Arnim, Z19 Herman Künne and Z21 Wilhelm Heidkamp lay magnetic mines in major shipping lanes in the English Channel. No notification is made, contravening International law and leading to catastrophic results the next day.