'Roundhammer' was an Allied unrealised plan, intermediate in scope between 'Round-up' and 'Sledgehammer', for an invasion of German-occupied northern Europe (1943).
The concept was adopted at the 'Trident' (3rd Washington) conference in May 1943 as a result of the urging of General George C. Marshall, the US Army Chief-of-Staff, that the Allies should concentrate their European effort on a selected area to ensure 'a decisive invasion of the Axis citadel'.
The result was 'Roundhammer', basically an improved version of 'Round-up' with an intended implementation date of 1 May 1944, by which time the USA proposed to have in the UK some 29 divisions. The object of the operation was the capture of a large lodgement in North-West Europe which could then be used as the basis for decisive operations against Germany as the assault forces were built up a rate of three to five divisions per month. From this basic concept there grew, via 'Skyscraper', the plan for 'Overlord' as the definitive plan for a cross-Channel landing, which matured in July and August 1943 before being finalised by the Allied leaders at the 'Quadrant' conference in Quebec.