After shakedown off Bermuda Halligan in mid-November joined Iowa carrying President Franklin D. Roosevelt and his party en route to the historic Teheran Conference.

She served as part of the escort screen to Casablanca, then for the next few weeks conducted anti-submarine operations off North Africa.

While guarding the transports, she survived almost continuous air attacks; at one point two bombs passed between her stacks and struck the water without exploding.

Later that same day, she departed Leyte to screen the battered escort carriers of "Taffy 3", withdrawing from their courageous battle against Japanese forces off Samar.

Halligan shot down one kamikaze 5 January, then sent rescue and repair parties to assist damaged Stafford.

On 17 February she served as lifeguard ship north of Iwo Jima during preinvasion air strikes.

Hit repeatedly by accurate gunfire, the attacker dropped a large bomb which landed about 100 yards (91 m) off the port bow and failed to explode.

Halligan closed to within 2,700 yards (2.5 km) of the shore of Iwo Jima 19 February; and, as the first wave of Marines headed for the beach, she joined other ships in providing a heavy support barrage.

Her guns destroyed a Japanese shore battery, and she spent much of D-Day pounding assigned target areas in support of the main landings.

She was soon underway again, this time as part of Task Force 54 (TF 54), steaming for the invasion of Okinawa—gateway to the heart of the Japanese Empire.