By that time, the German leadership believed that the United States was effectively acting as a belligerent in the conflict, given actions such as Lend-Lease of supplies to Britain to sustain their war effort, President Roosevelt's public statements, the deployment of American soldiers and Marines to Iceland, and U.S. Navy escorts of convoys across the Atlantic, which sometimes came into contact with U-boats; these acts, as well as America's previous intervention in World War I, led to the assumption that war between them was inevitable. As such, the decision was made to use the attack as a rationale for an official declaration of war in order to drive Britain out of the conflict by widening submarine operations and directly attacking U.S. commercial shipping. While Hitler's declaration of war against the United States eventually led to his downfall, initially it seemed successful in its objective of more effectively cutting Britain's supply lines, as the U.S. military's lack of tactics, equipment, and procedures for fighting U-boats caused 1942 to be the most devastating year of the war for shipping losses;[30] the war declaration enabled the Second Happy Time for U-boats.[31]