'Rankin' was an Allied unrealised series of three plans for large forces to move into northern Europe before the date set for 'Overlord' in the event that Germany underwent a sudden and drastic military or economic deterioration (1943/44).

In contingency planning for the European theatre, the Chief of Staff Supreme Allied Commander (COSSAC) team, headed by Lieutenant General F. E. Morgan, developed the 'Rankin' plan as a series of three variants designated as 'Rankin A', 'Rankin B' and 'Rankin C'. The trigger for any of these variants was to be a sudden change in Germany’s military strength and determination to continue the war.

'Rankin A' and 'Rankin B' were concerned with situations in which Germany was still fighting, albeit in a much weakened fashion. The requirement, therefore, was for Allied land and air forces to move from the UK into continental Europe as rapidly as possible and to undertake the final defeat Germany from there. 'Rankin C' was concerned with a movement onto the European continent in the event of a cessation of German resistance. In this eventuality, therefore, the requirement was to ensure the rapid occupation of the areas appropriate for the enforcement of the terms of unconditional surrender laid down by the Allied governments.

To a certain extent, 'Rankin' replaced earlier concepts such as 'Bolero', 'Sledgehammer' and 'Round-up' posited on an Allied return to the continent in 1943. While the British rejected this overall concept as premature without Mediterranean operations and strategic bombing, a variant of 'Round-up' had been considered as a contingency plan for a quickly launched invasion.

'Rankin A' dealt with the situation of 'substantial weakening of the strength and morale of the German armed forces' to the extent that a successful assault could be made by Anglo-US forces before the planned major invasion of Western Europe. If this took place before the end of 1943, no action was feasible unless it was clear Germany was close to collapse, and substantial naval resources were available. If the contingency took place after January 1944, the resources being concentrated for 'Overlord' could be used against weak opposition, and, after March, against stronger resistance. It was judged feasible to carry out 'Rankin A', which was essentially a modification of 'Overlord', after January 1944 against the Cotentin peninsula rather than Normandy, as long as the port of Cherbourg could be seized within 48 hours. Diversionary invasions might be needed in the Pas de Calais and in southern France.

'Rankin B' dealt with the contingency of German withdrawal from occupied countries, most probably Norway and France, in which case it would be necessary 'for political as well as strategic reasons, to occupy the areas vacated, but it [would also be] important that the main forces of the Allies should not thereby be tied down far from the eventual centre of action'. Different actions would be required for the two areas. For Norway, the goal would be establishing air and radar bases to protect the supply route to the USSR, and to block German break-outs from the Baltic sea. This could be done with one brigade in northern Norway and one division in southern Norway. If Germany withdrew from France, it would probably begin in Bordeaux and the western ports, with the Pas de Calais the last area to be evacuated. One brigade each was assumed needed for Bordeaux, Brest and Nantes, to prepare them from reinforcements coming directly from the USA. The main forcible entry at Cherbourg would be by US troops, while British troops would take Le Havre and Rouen. 'From these bases, the Allies would establish a line along the Somme [river] from which to press north-eastward through the Pas de Calais to Belgium, opening up ports as they proceeded and establishing airfields from which the advance could be covered and the attack on Germany itself concentrated. At the same time, Mediterranean forces would be required to occupy Marseille and Toulon, and to move northwards thence to [Lyons and Vichy].'

'Rankin C' dealt with possibilities such as a sudden German surrender, or at least cessation of organised German resistance. Had the Allied unconditional surrender policy not been in effect and the July 20 plot succeeded, 'Rankin C' might have been very relevant. It still could have applied if the plotters were successful in unseating Adolf Hitler and then surrendering Germany, perhaps with understandings about the retention of some authority. 'Rankin C' targeted the initial areas to come under control as the Jutland peninsula; the ports of Bremen, Hamburg and Kiel; and the large towns of the Ruhr and Rhine valleys. Using these as bases, the Allies would then occupy the entire Rhine and Ruhr valleys, and move into Denmark, Schleswig and Holstein. Air bases in these areas would be sufficient to establish air supremacy while the rest of Germany was disarmed. In parallel, selected ports on the European coast would be opened, and the capital cities of the occupied countries brought under control. Similar processes would be carried out in Norway and the Mediterranean. It was estimated that 24 divisions would be required for the primary areas of occupation: seven for Denmark and north-west Germany, six for the Ruhr and 11 for the Rhine valley. Other troops (non-field force formations wherever possible) would be required in support of the various national contingents charged primarily with the rehabilitation of their respective liberated countries.