One can imagine a scenario in which, in the absence of French and British support for Poland, the Danzig crisis of 1939 is resolved without war. In such a timeline, a combination of Soviet "salami tactics" (operations against Finland, the Baltic States, and Romania) and mounting evidence of Soviet plans for an invasion of Europe, cause Poland and Germany to put their (admittedly considerable) differences aside for the sake of dealing with the immediate danger.
In 1942, the Soviets feel strong enough to attempt the occupation of Romania. This convinces both the Germans and the Poles that their only choice is between fighting now (with the help of the Romanian armed forces and Romanian oil) and later (without such advantages.) They opted for the first.
(The trick now is to find some reason for the British and French to get involved. Having declined to go to war for the sake of Poland, it is hard to imagine them going to war in 1942 for the sake of the USSR.)