the Soviet Union invaded Poland from the East.
Why didn't the Allies declare war on the Soviets aswell?
For the same reason that made UK and US leave central and eastern Europe, Poland included, to Stalin during Yalta conference and sentence half of Europe to be raped, pillaged and screwed by communism for 50 years. Fear. Everyone feared Russia. Everyone still does.
In truth, everyone underestimated Hitler's Germany in thirties. And, purely on political level, both France and Britain, screwed up pretty bad and practically let the whole thing happen.
France was warned multiple times about Germany being in state of unrest and preparation prior to WWII. In 1933 The First Marshall of Poland discussed with France plans to organise preventive military action against Germany right after Hitler rose to power. France decided it was highly unlikely Germany would be of any threat to Europe and refused any help. Even after Germany violated Locarno treaty in Rhine region, which effectively forfeited Belgium's position as French ally, Poland offered (in accordance with existing agreements) full military support should France decide to stop the new forming Reich from gaining any further territories. But France once again declined and decided it was easier to fortify the border along the Maginote Line and let Germany know that way it was all serious business and all that.
Britain on the other hand, screwed up big time. In 1937 Edward Halifax, representing House Of Lords, met with Hitler and unofficially opened discussions about German demands of control over their old colonies, Austria, Sudetenland and passage/corridor to Danzig in Poland (effectively leaving Baltic to Germans). This was understood by Germany as permission from Britain to regain power in Central Europe and led to the "Anschluss" of Austria, loss of territories by Czechoslovakia and eventually the war as we know it. Not that the war wouldn't happen without the British "appeasement", but it definitely sped the process up.
Three days after attack on Poland Britain and France declared war on Reich in accordance with treaties signed almost at the last moment, in summer of 1939. Britain was more or less hoping that the whole thing would just blow over and Germany wouldn't dare to proceed any further in fear of allied military action. You can kind of see how when the major military forces in Europe threaten relatively poor economically country like Germany, they would expected it to have an immediate effect. For both France and Britain it was also logical that Russia at some point would join the forces with "the majority" against Germany once declaration of war was issued. But Germany did not care. And Russia had its own plans for Europe.
Despite popular belief, Poland was actually relatively well prepared for the war. Just not that kind of war. They based their entire defences on WWI experience, with infantry geared for close combat and plenty of horse units equipped with short weapons against infantry. However, Blitzkrieg brought completely different technology to the table and the masses of single shot rifle and bayonet equipped infantry, heavy and medium horse cavalery, horse mounted rifles and chevau-légers (lancers) practically got decimated in matter of days by German units arriving with heavy, medium and light machine guns, armoured vehicles, tanks and both sea and land based artillery barrages executed with high precision and extending for days. Nobody was prepared for that type of war. Not Poland, not France, not even Britain, if they were to face that enemy on land in 1939. This was no ordinary gentleman's war led by old Prussian Generals. There would be no sitting in foxholes for months. No more capture the flag games. No 5 o'clock tea breaks. It was fast, mechanised, high tech and absolutely ruthless.
Without Russian support and with Belgium, Holland and Luxemburg remaining neutral, Britain and France screwed up once more. 12 days after attack on Poland Neville Chamberlain, Edouard Daladier and Maurice Gamelin met in Abbeville and decided (despite the terms of treaty with Poland) not to action any military offensive against Germany, but to "prepare, wait and see" what happens instead. The entire strategic defence of Poland in September 1939 was based on assurance that by day 15 the allies would come to rescue. The help never came. Defences were decimated and run out of ammo. Russia did the rest on day 17. The rest, as they say, is history.