The prehistory is a fascinating episode in its own right, in which initial "can-do" US ebullience ran into pragmatic British caution and Churchillian leanings toward a peripheral strategy.

Curiously given later British reluctance, the plan originated in London at the end of 1941 as a blueprint for a 1943 landing around Le Havre "under conditions of severe deterioration of German military power". The name of Operation Roundup reflected this vision of action to exploit incipient enemy collapse, though given the limited size of the force then expected to be available there was little beyond a dash through the Low Countries to northwestern Germany, (though this interestingly remained the British task in 1944) and the option wasn't given any immediate priority.

The proposal was dusted off and passed to Washington on 16 March 1942 just as the US Joint Chiefs were agreeing to prioritise a concentration of forces in Britain for a "Germany first" strategy. The US Army's Operations Division (until four days earlier the War Plans Division) came up with a more ambitious scheme on March 27, approved with minor modifications by army chief-of-staff Marshall, war Secretary Stimson and Roosevelt at the beginning of April, taken by Marshall to London a week later (as now the "Marshall memorandum") and approved by the British on April 14 with the proviso of adequate forces in the short term to defend India from the Japanese advance through Burma.

The US plan comprised three elements: Operation Bolero, the procurement and assembly of up to a million US troops in Britain by April 1943; a spring 1943 landing between Le Havre and Boulogne (again east of the eventual beaches), for which the Roundup name was retained; and Operation Sledgehammer, a possible autumn 1942 contingency landing "if it should be absolutely necessary to prevent a collapse of Soviet resistance or if the German position in western Europe had become critically weakened".

While the British were in agreement on Bolero and (apart from worries over landing craft and long-range fighters) Roundup (which had after all been their idea), differences arose immediately on the 1942 Sledgehammer contingency: the British felt that the Channel weather made August the latest realistic date, while their US opposite numbers feared there wouldn't be enough troops on hand until mid-September at the earliest; the British also resisted a 1942 emergency landing, urging that it should only be undertaken in the event of German rather than Soviet collapse, though the matter was left to the unfortunate planners whose job really wasn't to be taking such decisions.

Sledgehammer is now widely considered an operation too far, and August's Dieppe raid was to offer a glimpse of some of the difficulties involved; but its planning fed into the bigger Roundup operation even as prospects of a 1942 invasion waned amid deepening British doubts about the availability of landing craft. If an opportunistic assault was to be undertaken amid German collapse, the British reasoned, air cover was less of a priority and a Pas-de-Calais landing against stronger defences could be abandoned in favour of better ports, namely Cherbourg or Le Havre, with the latter preferred as it offered better airfields. (This of course recalls the original British Roundup.)

Sledgehammer's woes deepened with the painful British reverse in Libya and Egypt in June. News of Tobruk's fall came through just as Churchill was meeting Roosevelt on the 21st, and the PM at once threw his weight behind the alternative Allied strategy (agreed at Washington the previous winter) of a US-led landing in Vichy-held French North Africa to squeeze Rommel from the southern shore of the Mediterranean. The two leaders agreed that while a cross-Channel invasion would "yield greater political and strategic gains than operations in any other theater", strategic uncertainties necessitated preparing an alternative - the North African operation, codenamed Gymnast.

On July 6 the British Chiefs "unanimously agreed that operation Sledgehammer offered no hope of success, and would merely ruin all prospects of 'Roundup' in 1943", and Washington was so notified on the 8th. US strategists on the other hand feared that a Mediterranean entanglement in 1942 would erode the chances of a landing in France even in 1943. Marshall proposed that the US now shift its focus to the Pacific, but was sensibly overruled by Roosevelt who saw how such "abandonment" would be received in London (and doubtless Moscow).

Marshall was ordered back to London with navy C-in-C King and presidential aide Hopkins to sort the mess out, with Sledgehammer still the preferred US strategy for 1942 but authorisation to determine "another place for US troops to fight" if there was no progress. At their July 20 meeting US fears were realised when Churchill even proposed to scale down the 1943 Roundup operation, though Eisenhower (commander of US forces in Europe since June 24) himself rated Sledgehammer's chances of success at best one in two.

On July 22 the Americans wearily accepted defeat in the face of a final British no to Sledgehammer. At Marshall's request, Eisenhower drew up an outline 1942 North Africa strategy which was agreed by the US delegation and transmitted to Washington. The Combined Chiefs adopted the new strategy on July 24, and Gymnast became Operation Torch (with its own fascinating prehistory to its execution on November 8). The subsequent tale of how a speedy takeover of North Africa became bogged down into a six-month slog to Tunis and the curtain-raiser to the similarly drawn-out Italian campaign needs no re-telling: suffice it to say that in 1943 Roundup gave way to Operation Overlord, provisionally scheduled for May 1944, conforming US fears that a Mediterranean "sideshow" would delay the main event.

It should be noted in fairness that the risks of an at best hazardous 1942 cross-Channel landing would have fallen heavily on the British, with only 200,000 US ground troops projected to be available in Europe by the end of the year, so to some extent it was the UK's call. But the eventual derailment of even a 1943 operation involving far greater US numbers left a bitter taste among Britain's allies who rightly doubted the allure of a Mediterranean "soft underbelly" that turned out to be anything but.

The story is told in Ray S Cline, Washington command post: the Operations Division, US Army Center for Military History 1990, and Gordon A Harrison, Cross-Channel attack, Center for Military History 2002, and engagingly in Harry C Butcher's My three years with Eisenhower, Simon & Schuster 1946.