why partition of poland between SU and Germany is the start of the war while partition of Czechoslovakia between Nazi Germany and Poland is not?
Because the nazi invasion of Czechoslovakia, and Polish regaining of Zaolzie (stolen from us twenty years earlier by the Czechs while we were busy fighting the Soviet invasion), didn't trigger off WW2. It didn't provoke France and Great Britain to declare war on Nazi Germany, hence it wasn't the beginning of WW2.
Simple as that.
The fact is, Kostya, you were enthusiastically licking Hitler's anus and even in June 1941, when your lover and friend finally decided to end the romance, Stalin couldn't believe what was happening at first.