Puppet governments controlled by USSR. When you sign a document under duress it is invalid.
You are right if we talk about treaties signed under Civil law. However as pointed out to you several times, international law does consider this as an argument. And it certainly does not consider this as an argument when succesive governments acknowledge the treaty. Otherwise all peace treaties would be invalid. Political pressure is not an argument for declaring a treaty invalid.
That's how court works were preparing our case as plaintiff, Germany as defendant.
That is not how international law works. Germany does not have to defend anything, Berlin will simply point to the existing treaties. It is up to Poland to prove that those treaties are somehow invalide, something that usually has a very slim chance of success I might add. Just to give you an example:
Germany and the Czech Republic currently disagree about the nature of the Munich agreement (and the aftereffect, when the president of the CSR had to agree to the dissolution of his country.). Both declared it as void, but for Berlin the Agreement was only nullified after it was broken by Hitler, whereas Prague considers it as void from the beginning. Yet Prague never went to court over it, even though we know that actual physical pressure was applied on the signatory party (the very ill Czech president had to wait for hours and was heavily pressured and close to a heart attack when he signed it), because - aside from the fact that it would be a needless provovation - as Czech lawyers have pointed out, it is far from clear that prague could win.
And mind you this treaty was never confirmed by succesive - including democratic - governments and we know that not only political pressure was involved. Treaties are almost never declared invalid without the approval of all signatories, especially after such a long time. Those are the facts.
@Dirk diggler: I'd suggest that you read a bit about international law before you continue this debate. You are right, none of us are (probably) lawyers, but there are certain fundamentals everybody can understand when read. And unfortunately, it seems that you either are oblivious of them or fail to acknowledge them. Otherwise you would know that your cited arguments ("puppet government" "against the will of the people") have no relevance here.
Remember, international law does not apply exlusively to democratic states, but is supposed to be applied to all countries in the world. Many of them are neither democratic nor completely independent (not that "independent" is really verifiable in this multilateral age).