Okay, declare war on Germany, kick their ass, and force their withdrawal from the country. That’s all well and good, for the first couple weeks. Then the Soviets take the rest of it. So once they got the Nazis out, how were they planning to deal with the Soviet part of Poland? They didn’t help Finland significantly, not that they needed much, when the Soviets invaded. They weren’t willing to go to war when they took the rest of Eastern Europe. Given that I’m guessing they wouldn’t have been willing to war over Poland either (barring Patton and a select few others), but I’m wondering if there were any actual plans in place one way or the other.
Who’s “they”?
The British and the French who were Poland’s nominal allies and who declared war on Germany for invading Poland.
The Soviets obviously were willing to take advantage of the Germans’ initial willingness to compromise and divide spoils like Poland.
Remember that the borders between various european states were pretty arbitrary, some empires used to “own” other countries, and in general the divisions made after WWI were designed to punish the losers; that’s why Germany was upset - chunks of land they thought should be part of greater Germany were in Poland, Czechoslovakia, and France.
Hitler’s first preoccupation was to take back these areas. The extra pieces that came with them were a bonus. I’m sure he realized that taking back Alsace - Lorraine would eventually precipitate another Franco-Prussian war, where he hoped to kick French ass like in 1870. He was hoping that the British had no inclination to fight, France would not go it alone on Poland, so it would be free for the taking and he could gear up for a solo match with France over the western border.
As for the Soviets - Hitler probably realized it would be easier to deal with France first, then worry about the east… Essentially what he did do. Whether he really planned to take over the world in 5 years, or whether once the avalanche started, he simply had to ride it all the way… I don’t know what his actual thinking was. Initially, he just wanted to reunite the fatherlands.
The Soviets? They probably did not plan to take on Germany all by themselves, and realized they did not have the military strength to be aggressive. They knew they could expect no help from France and England; they were just lucky that afterwards they fell into “enemy of my enemy” category. Until they were attacked, they were simply picking over any carcasses Germany left in their way.