Most of any offensive operation would have been undertaken by American and British forces, as well as Polish forces and up to 10 divisions of the former German Wehrmacht, remobilised from POW status. Any quick success would be caused by surprise alone. If a quick success could not be obtained before the onset of winter, the assessment was that the Allies would be committed to a protracted total war. In the report of 22 May 1945, an offensive operation was deemed "hazardous". In response to an instruction by Churchill of 10 June 1945, a follow-up report was written on "what measures would be required to ensure the security of the British Isles in the event of war with Russia in the near future". US forces were relocating to the Pacific for a planned invasion of Japan, and Churchill was concerned that the reduction in supporting forces would leave the Soviets in a strong position to take offensive action in Western Europe. The report concluded that if the United States focused solely on the Pacific Theatre, Britain's odds "would become fanciful". The Joint Planning Staff rejected Churchill's notion of retaining bridgeheads on the Continent as having no operational advantage. It was envisaged that Britain would use its air force and navy to resist, but a threat from mass rocket attack was anticipated, with no means of resistance except for strategic bombing. By 1946, tensions and conflicts were developing between Allied-occupied and Soviet-occupied areas of Europe. They were seen as being potential triggers for a wider conflict. One such area was the Julian March (an area of southeastern Europe now split among Croatia, Slovenia and Italy), and on 30 August 1946 informal discussions took place between the British and US chiefs of staff concerning how such a conflict could develop and the best strategy for conducting a European war. Again, the issue of retaining a bridgehead on the continent was discussed, with Dwight D. Eisenhower preferring a withdrawal to the Low Countries, rather than Italy, because of their proximity to the United Kingdom. In June 1945 senior Soviet Army commander Marshal Georgi Zhukov suddenly ordered Soviet forces in Poland to regroup and prepare their positions for defence. According to Edinburgh University professor John Erickson, Operation Unthinkable helps to explain why he did it. If the plans of the operation had been transmitted to Moscow by the Cambridge Five, that would explain the sudden orders to regroup and prepare for defence, however, it is just as possible that it was Soviet mistrust of Western Allies. If the Soviets had indeed known that the Western Allies were planning a possible attack, the element of surprise would have been lost before operations against the Soviets even began, further reducing the chances of Operation Unthinkable's success.