The Tehran Conference in November 1943 was important because

Churchill Stalin and Roosevelt agreed on creation of second front in Europe as soon as possible, support for Yugoslav partisans and agreed that due to fact that Russia will keepo Polish eastern territories Poland whill gain Masuria and Ermland and whole Upper Silesia (Polsih speaking German territories) and Lower Silesia with Pomerania (which were part of Poland centuries earlier) as compensation for lost lands in the east