'Indigo III' was a US naval operation to deliver reinforcements and equipment to bolster the US Marine Corps garrison of Iceland (12/16 September 1941).

The British had occupied Iceland from 10 May 1940 in 'Fork' followed by 'Alabaster', but by the middle of 1941 needed the men of their garrison for service elsewhere and, in July 1941, passed responsibility for Iceland to the USA under a US/Icelandic defence agreement. President Franklin D. Roosevelt ordered the US occupation of Iceland on 16 June 1941. Brigadier General John Marston’s 1^st Provisional Marine Brigade of 194 officers and 3,714 men from San Diego, California, sailed from Charleston, South Carolina, on 22 June to assemble for movement by Task Force 19 from Argentia, Newfoundland, in the transports Heywood, Fuller, William P. Biddle and Orizaba, the attack cargo ship Arcturus, the destroyer tender/cargo ship Hamul, the fleet oiler Salamonie and the fleet tug Cherokee, which were escorted by the battleships Arkansas and New York, the light cruisers Brooklyn and Nashville, and the destroyers Lea, Upshur, Bernadou, Ellis, Buck, Benson, Mayo, Gleaves, Niblack, Lansdale, Hilary P. Jones, Charles F. Hughes and Plunkett.

TF19 departed Argentia on 1 July, and on 7 July the British persuaded the Althing (Icelandic parliament) to approve a US occupation force. TF19 anchored off Reykjavík during the evening of 7 July. The 1^st Provisional Marine Brigade started to come ashore on the following day, and disembarkation was completed on 12 July. On 6 August, the US Navy established an air base at Reykjavík with the arrival of the VP-73 patrol squadron with Consolidated PBY flying boats and the VP-74 patrol squadron with Martin PBM Mariner flying boats. US Army personnel began arriving in Iceland during August in 'Indigo III', and the marines had been transferred to the Pacific by March 1942.

On 6 August the US Navy established Rear Admiral William R. Munroe’s TF16 with the fleet carrier Wasp, battleship Mississippi, heavy cruisers Quincy and Wichita, and five destroyers to escort the transports Almanack, American Legion and Mizar to Iceland.

Munroe, now commanding TF15 with the battleship Idaho, heavy cruisers Tuscaloosa and Vincennes, and destroyers Winslow, Anderson, Mustin, O’Brien, Sampson, Benson, Hilary P. Jones and Niblack, provided cover for a convoy of four transports carrying one US Army brigade, three freighters, the fleet oiler Cimarron, workshop ship Delta, and an escort group with the destroyers Bainbridge, Overton, Reuben James, Truxtun, MacLeish, Walke and Morris.

During 11/12 September the destroyers undertook several depth charge attacks in prosecution of eight sonar contacts, though all of these were later revealed as false, but on 14 September Truxtun sighted a U-boat and Truxtun, MacLeishand Sampson depth-charged the boat as it dived.

On 16 September TF15 and the convoy anchored in the Hvalfjörður near Reykjavik in south-western Iceland.