Germany used its improved submarines to establish a blockade of British ports in an attempt to force Britain to surrender by destruction of the shipping on which it depended for food and other supplies. Their use against American ships proved to be a damaging tactic.
April: The United States government warned it would defend its right to trade and the Americans joined the Allies.
German naval command decision to use unrestricted submarine warfare almost succeeded in forcing Britain to sue for peace within months. British shipping losses soared to 875,000 tons per month by April. Merchant sailors on British and neutral vessels began to refuse to sail.
May 10: At the insistence of Prime Minister Lloyd George the convoy system was adopted. British escort vessels, joined by American destroyers in May, provided protection to merchant ships and at the same time were able to sink more submarines. The convoys were an immense success. Although shipping losses by the end of the year exceeded eight million tons, Allied shipbuilding programs more than offset the losses.