Final answer:
Destroyers were first used effectively for antisubmarine warfare during World War I.
Step-by-step explanation:
The first effective use of destroyers for antisubmarine warfare occurred during World War I. The introduction of submarines in this war led to a new era of naval combat, as they could sink unsuspecting vessels with torpedoes. The German navy, known for its U-boats, focused on expanding its submarine fleet during this time. The Allies struggled to defend against these submarine attacks until the rapid addition of American naval escorts and the establishment of a convoy system.