France and Germany negotiated terms of armistice June 22, 1940. France formed a new government, was allowed to maintain its own civil service and police and keep some of it's army intact. The French army fought allied forces in North Africa and the Levant as a German ally. While the term were harsh the Germans did not require "unconditional surrender," but allowed Vichy France to continue as a political entity. What motivated the US, Brits, Soviets and Chinese to give the French victor nation status and a permanent seat on the UN security council? Why were Norway, Holland, Denmark not given similar status?
Because of the significant number of Free French Forces which fought with the Allies in Africa, Italy, and Europe. De Gualle was also very vocal about his contribution, once he had wrangled what amounted to both political and military leadership of the FFF.
They also began the war as an ally and were on equal terms in prosecuting the war with Britain until the surrender.
Churchill and the UK foreign office really pushed for greater French inclusion in post-war Europe. In their calculations, France would be a good continental partner in the face of a revived Germany, or more immediately, the USSR. It must be remembered that Churchill and the UK as a whole were unsure about the US policy after world war II. The US had not made any concrete guarantees against the Soviet Union until the creation of NATO.
Additionally, the greatest divide in US and UK foreign policy was extra-European and economic matters. The US was very much in favour of decolonisation and freeing markets for US goods. France, itself an imperial power like the British were a useful body to include in the UN with a veto.
For the British in 1944 and 1945 it was very possible that it would be alone, in checking the threat of the USSR and erosion of empire. Having a friendly, like-minded nation in powerful institutions was strategically useful to the UK.