Tens of thousands of Polish Jews settled in Germany in the inter-war period due to nationalist and racist policies in Poland. This fostered still more anti-Semitic hatreds in Germany and to a mentally ill like Hitler this was unforgivable.
Poland had a considerable Jew community, which Hitler wanted to eliminate.
To a nationalist like Hitler, the lost territories by Germany in WWI in favour of Poland were unacceptable. But above all, Hitler wanted to push Germany into war to control the whole world and Poland was a first step because UK and France guaranteed Polish independence.
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Soviet Russia
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Poland (Jews)
The Main one was Poland
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