Even so, by early October the Allies were growing increasingly confident at the effectiveness of their blockade and the apparent success of the recently introduced convoy system. A convoy of 15 freighters arrived in British ports unscathed from Canada bringing half a million bushels of wheat, while in France more important ships arrived from Halifax in another convoyed group. The French claimed that of 30 U-boats sent out in Germany's first major offensive against Allied shipping, a third had been destroyed, and Churchill declared that Britain had seized 150,000 more tons of contraband than was lost by torpedoing. In mid-October Adolf Hitler called for fiercer action by his U-boat crews and the Luftwaffe to enforce his counter-blockade, and warned the Allies of his new "secret weapon". Neutral ships were warned against joining Allied convoys, Scandinavian merchants were ordered to use the Kiel Canal to facilitate the German's own Contraband Control and the US City of Flint, which had rescued survivors of the Athenia became the first American ship captured as prize of war by the Germans, although the episode proved farcical and the ship was eventually returned to its owners.
