The Commonwealth had been forced to rely on Russia for protection against the rising Kingdom of Prussia , while Prussia was demanding a slice of the northwest in order to unite its Western and Eastern portions , although this would only leave the Commonwealth with a Baltic coast in Latvia and Lithuania . The Commonwealth had been forced to rely on Russia for protection against the rising Kingdom of Prussia , while Prussia was demanding a slice of the northwest in order to unite its Western and Eastern portions , although this would leave the Commonwealth with a Baltic coast only in Latvia and Lithuania .