Molotov%E2%80%93Ribbentrop Pact
At the same time, British, French, and Soviet negotiators scheduled three-party talks on military matters to occur in Moscow in August 1939, aiming to define what the agreement would specify should be the reaction of the three powers to a German attack. The tripartite military talks, started in mid-August, hit a sticking point regarding the passage of Soviet troops through Poland if Germans attacked, and the parties waited as British and French officials overseas pressured Polish officials to agree to such terms. Polish officials refused to allow Soviet troops into Polish territory if Germany attacked; as Polish foreign minister Józef Beck pointed out, they feared that once the Red Army entered their territories, it might never leave.

Q: Why did Poland deny the proposal of the Soviet Union protecting them from a German Attack?
once the Red Army entered their territories, it might never leave.