It's the simplest and most natural option.
...to us. It could have been done in 1919 already, but stalled. Prussia didn't make any sense back then already. Neither did Lippe-Detmold.
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On Sovjet intervention- how about Teschen being an excuse for a DOW? What if the Polish burn their fingers?
Because, in any other way I cannot see a meaningful internvention of the Red Army.