'Barrier' was an unsuccessful Allied sea patrol in the South Atlantic between South Africa and South America to intercept German blockade-running freighters (1/12 December 1943).
The launch of this operation was triggered by the despatch, on 25 November, of the codeword 'Kammerarrest' by the U-boat command to boats operating in the South Atlantic. The codeword forbade attacks on single merchantmen as from 1 December, and when decoded by the Allies on the following day served to confirm the imminent arrival of the first of an expected five blockade runners known to be voyaging from Japan.
This in turn led to the activation of 'Barrier' patrols involving Rear Admiral Albert C. Read’s Task Force 41 comprising five task groups each comprising one cruiser and one destroyer, US Navy and Brazilian air squadrons from Natal and Recife, and air patrols of US patrol bomber squadrons from Ascension island, beginning on 1 December. 'Freecar' was also launched a few days later from Freetown and Dakar with a number of British, French and Italian co-belligerent vessels.
The first of the blockade runners, the 6,951-ton Osorno, had left Kobe on 2 October, rounded the Cape of Good Hope on 15 November, and was sighted on 8 December by a Consolidated Liberator of the US Navy’s VPB-107 squadron based on Ascension island, but the searching TG41.4 (light cruiser Marblehead and destroyer Winslow) managed to intercept only an Allied freighter while Osorno, disguised as the British Prome, escaped.
A search to the north-west yielded no result, but U-510, on passage to the Indian Ocean and having sighted Osorno, was reported by a Liberator from Ascension and hunted on 11/13 December by TG41.3, comprising the light cruiser Memphis and destroyer Somers reinforced by the destroyer Jouett from TG41.1. On 13 December the Allies decoded U-510's signal about the sighting of Osorno, and decided that the blockade runner has already broken past the 'Barrier' and 'Freecar' patrols, which were therefore suspended.
This helped the following blockade runner, 2,170-ton Alsterufer, to pass through the narrows without being detected.
On 12 December the British began the follow-on 'Stonewall' operation in the North Atlantic.