Few reasons: State was defunct few hours earlier due to government and military command running away to Romania (before they left they also gave explicit order to not engage Soviet forces, which was followed). By the way, interwar Poland and even Polish London government was never in a state of war againt USSR, the latter just didn’t had relations with USSR from its formation in 1939 to 1941.
Next reason was to get back the ethnically belarussian and ukrainian lands invaded and annexed by Poland in 1919-21.
Finally, after numerous tries at containing rising nazi threat and utter lack of doing anything by UK and France, USSR decided to grab buffor before inevitable nazi invasion. Politically it was immediately bad move, but in the hindsight after analysing the operation Barbarossa, it most likely (together with Baltics) gave Red Army enough time to mount defense and prevent capture of Leningrad, Moscow and Stalingrad which would in all likeliness lost the war.
But, like, why did the Soviets invade Poland at the time?
Few reasons: State was defunct few hours earlier due to government and military command running away to Romania (before they left they also gave explicit order to not engage Soviet forces, which was followed). By the way, interwar Poland and even Polish London government was never in a state of war againt USSR, the latter just didn’t had relations with USSR from its formation in 1939 to 1941.
Next reason was to get back the ethnically belarussian and ukrainian lands invaded and annexed by Poland in 1919-21.
Finally, after numerous tries at containing rising nazi threat and utter lack of doing anything by UK and France, USSR decided to grab buffor before inevitable nazi invasion. Politically it was immediately bad move, but in the hindsight after analysing the operation Barbarossa, it most likely (together with Baltics) gave Red Army enough time to mount defense and prevent capture of Leningrad, Moscow and Stalingrad which would in all likeliness lost the war.
They didn’t: https://espressostalinist.com/2013/11/06/grover-furr-did-the-soviet-union-invade-poland-in-september-1939-the-answer-no-it-did-not/