In mid-1941, before the United States entered World War II, Idaho and her sisters were sent to join the Neutrality Patrols that protected American shipping during the Battle of the Atlantic.

She shelled Japanese forces during the Gilbert and Marshall Islands and the Philippines campaigns and the invasions of Peleliu, Iwo Jima, and Okinawa.

The ship was powered by four-shaft Curtis turbines and nine oil-fired Babcock & Wilcox boilers rated at 32,000 shaft horsepower (24,000 kW), generating a top speed of 21 knots (39 km/h; 24 mph).

[1] The ship was armed with a main battery of twelve 14-inch (356 mm)/50 caliber guns[Note 1] in four, three-gun turrets on the centerline, placed in two superfiring pairs forward and aft of the superstructure.

The secondary battery consisted of fourteen 5-inch (127 mm)/51 caliber guns mounted in individual casemates clustered in the superstructure amidships.

In addition to her gun armament, Idaho was also fitted with two 21-inch (530 mm) torpedo tubes, mounted submerged in the hull, one on each broadside.

Her original turbines were replaced with new geared models manufactured by Westinghouse, and she received six express boilers designed by the Bureau of Engineering.

Her armament was also revised, with the main battery turrets being modified to allow elevation to 30 degrees, greatly extending the range of the guns.

[1] Shortly afterward, the ship began her shakedown cruise, departing on 13 April for Guantánamo Bay, Cuba, before returning to New York.

The fleet then conducted a series of training exercises and held a naval review on 13 September for President Woodrow Wilson.

The ship was also present for a variety of ceremonies during this period, including a Naval Review for President Warren Harding in Seattle in 1923.

For the next six years, Idaho was based in San Pedro, where she continued to conduct readiness training, alternating between the Pacific and the Caribbean Sea.

[8] The lengthy reconstruction finished on 9 October 1934, and after completing another shakedown cruise in the Caribbean, Idaho returned to San Pedro, arriving on 17 April 1935.

[9] On 7 May 1941, Admiral Harold Stark, the Chief of Naval Operations, transferred Idaho, her sisters Mississippi and New Mexico, the aircraft carrier Yorktown, four light cruisers, and two destroyer squadrons to the Atlantic to reinforce the Neutrality Patrols.

[8] With the United States now an active participant in World War II, Idaho and Mississippi left Iceland on 9 December to rejoin the Pacific Fleet.

On 7 September, Idaho returned to San Francisco to begin preparations for the next major amphibious assault, which shifted focus to the central Pacific.

Idaho replenished fuel and ammunition at Majuro before returning to shell Japanese positions on other islands in the Marshalls before steaming to Kavieng, New Ireland to conduct a diversionary bombardment on 20 March.

During the Battle of the Philippine Sea on 19–20 June, Idaho remained with the invasion fleet and protected the troop transports and supply ships.

In the ensuing Battle of Peleliu, dug in Japanese defenders inflicted heavy casualties on the assaulting Marines, with Idaho providing artillery support until 24 September, when she withdrew for an overhaul.

There, she joined the bombardment group, which proceeded to the Marianas and then on 14 February steamed north to Iwo Jima, the target of the next major amphibious assault.

The ship bombarded the Japanese defenders on 19 February as the Marines went ashore; Idaho remained there for nearly a month before withdrawing on 7 March to replenish at Ulithi.

[8] On 21 March, Idaho joined Task Force 54 (TF 54), the Gunfire and Covering Group under the command of Rear Admiral Morton Deyo, as the flagship of Bombardment Unit 4 for the invasion of Okinawa.

Idaho shot down five kamikazes in a massed attack on 12 April, and in return, a near miss inflicted damage to her port side anti-torpedo bulge.

Permanent repairs were completed quickly, allowing the ship to return to Okinawa on 22 May, where she resumed her fire support mission.