I have read in The Maisky Diaries: Red Ambassador to the Court of St. James's 1932-1943, edited by Gabriel Gorodetsky, that on the afternoon of 29 March 1939 the British foreign secretary, Lord Halifax, received "intelligence reports from Berlin about an impending German attack on Poland."
What if there was such an attack? Would Britain and France declare war on Germany? As there was no Nazi-Soviet Pact, I assume that the Soviet Union would not invade eastern Poland and therefore Germany would take longer than in OTL to conquer Poland. Or would Poland successfully resist German invasion?