During the next few years Monaghan operated primarily in the North Atlantic, training the officers and men of the Navy who served the Nation so well in World War II.
On 7 December 1941, Monaghan was ready duty destroyer in Pearl Harbor, and at 0751 was ordered to join Ward who had just sunk an unidentified submarine off the entrance to Pearl Harbor. Four minutes later, before Monaghan could get underway, the Japanese air attack began. Monaghan opened fire, and at 0827 was underway to join Ward when notified of the presence of a midget submarine in the harbor. Monaghan headed for the trespasser, rammed it glancingly, then sank it with two depth charges. She headed on out of the harbor to patrol offshore for the next week, then joined Lexington in the attempt to relieve doomed Wake Island, captured by the Japanese before Lexington's force could bring aid. Homeward bound, Monaghan, with Dale and Aylwin, made repeated attacks on an enemy submarine, causing it to broach and give off a large oil slick. (continued)