On 12 September 1944, King Peter II called on all Yugoslavs to come together under Tito's leadership and stated that those who did not were "traitors", by which time Tito was recognized by all Allied authorities (including the government-in-exile) as the Prime Minister of Yugoslavia, in addition to commander-in-chief of the Yugoslav forces.
Churchill met Roosevelt at the Second Quebec Conference (codename Octagon) from 12 to 16 September 1944. Between themselves, they reached an agreement on the Morgenthau Plan for the Allied occupation of Germany after the war, the intention of which was not only to demilitarise but also de-industrialize Germany.
The Lapland War was fought between Finland and Nazi Germany effectively from 15 September 1944 to 27 April 1945 in Finland's northernmost region, Lapland. The Wehrmacht successfully withdrew and Finland upheld its obligations under the Moscow Armistice, although it remained formally at war with the USSR and the United Kingdom until ratification of the 1947 Paris Peace Treaty.
The UN was formulated and negotiated among the delegations from the Allied Big Four (the United States, the United Kingdom, the Soviet Union, and China) at the Dumbarton Oaks Conference from 21 September 1944 to 7 October 1944 and they agreed on the aims, structure and functioning of the UN.
An attempt to advance into northern Germany spearheaded by a major airborne operation in the Netherlands failed. Operation Market Garden was a failed military operation fought in the Netherlands from 17 to 25 September 1944.
On 28 September 1944, the Telegraph Agency of the Soviet Union (TASS) reported that Tito signed an agreement with the Soviet Union allowing "temporary entry" of Soviet troops into Yugoslav territory, which allowed the Red Army to assist in operations in the northeastern areas of Yugoslavia.