POD: Instead of becoming overconfident after the decapitation of much of the Iron Guard's leadership in November 1938, Prime Minister Armand Calinescu and King Carol order another purge of the Iron Guard and their supporters in the April of 1939. Horia Sima and Ion Antonescu are executed for treason.
April-July 1939: Calinescu ensures that the Iron Guard will no longer trouble Romania by conducted a ruthless extermination campaign against the Iron Guards. He also cements ties with Poland, signing a mutual defense pact in June. These actions do not endear him to Hitler, and several assasination attempts orchestrated by German operatives fail. While Calinescu publicizes these assasination attempts as best he can, nothing will deter Hitler from pursuing a path of aggression.
August 23, 1939: The Nazi-Soviet Non-Aggression Pact occurs on schedule. Nazi Germany agrees to a partition of Poland slightly more favorable to the Russians in exchange for having all of Romania except Bessarabia included in the German sphere of influence.
August 24, 1939: Poland and Romania begin mobilization to defend against what is obviously an imminent threat. Unfortunately, guarantees of Romanian assistance ensure that the Polish armies are deployed even farther west along the Polish Corridor when the war begins.
August 26, 1939: Encouraged by Hitler, Hungary begins mobilizing it troops and sending them towards the Romanian border.
August 30, 1939: The first Romanian units begin entering Poland.
September 1, 1939: The German invasion of Poland begins.
September 3, 1939: Romania declares war on Germany.
Differences from OTL:
Germany
Hitler is determined to defeat upstart Romania immediately after finishing off Poland. As a result, all of the Group reserves from Army Group North (1 panzer division, 3 infantry divisions) have been pulled to the front lines of Army Group South, and VII Corps (OTL reserve for Army Group South, 2 infantry divisions) will be right in the thick of things from the get go.
Poland
The Karpaty Army has been reinforced by the elite Romanian 2nd Mountain Brigade. The reserve Lublin Army has had the Romanian 2nd Infantry Division added. Prusy Army (the General Reserve) has been increased by the addition of the Romanian 14th Infantry Division, plus all of its units have been mobilized, as opposed to OTL where it was disorganized and never fully mobilized. In addition a "Lwow Army" consisting of the Romanian 6th and 16th Infantry Divisions as well as four cavalry brigades is in position at Lwow, in southeastern Poland.
Romania
In addition to the four infantry divisions, four cavalry brigades, and mountain brigade deployed in Poland, ten infantry divisions, five cavalry brigades, and an armored regiment have been stationed to defend against the impending Hungarian attack. Three infantry divisions and two cavalry brigades are stationed in Bessarabia and Bukovina to defend against any Soviet "stab", while four infantry divisions and two cavalry brigades are concentrated around Bucharest as a reserve group.
Hungary
With mobilization nearly complete, Hungary is poised to begin an offensive into Romania at the earliest favorable opportunity.
USSR
The USSR is preparing to assault Poland once a German victory is ensured, as in OTL. Stalin intends to wait until after Germany has done the hard work in subduing Romania before sweeping in and occupying Bessarabia.
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So, any ideas on what happens now? I personally could see a very ill-timed offensive by the Poznan Army into Germany after a couple of days, since the progress of Army Group North will be slowed down by the increase of forces in Army Group South and an overall increase in Polish troops as they are already mobilized. The extra advantages Poland has due to being prepared earlier and aided by an ally may be nullified by a false sense of security, enacted upon by incompetent strategic decisions.