My arguments are the same as those laid out in the first comment by AES_Enjoyer, as are those of Denna.

Importantly, you never stated what those "several alternatives" are.

Actually, the Nazi determination to settle accounts with Poland had for months been as plain as anything could be. This time, too, only the most heroic measures could prevent them from taking what they wanted in an orgy of violence and blood-letting. They had been frustrated at Munich, prevented from trying out their new war machine. Now they were determined to see for themselves just how much destruction it could cause. When the Italian Foreign Minister, Count Ciano, talked with Hitler and Ribbentrop on August 11–13, he wrote in his diary: “The decision to fight is implacable. . . . I am certain that even were the Germans given much more than they ask, they would attack just the same because they are possessed by the demon of destruction. . . . There is nothing that can be done. Hitler has decided to strike and strike he will.” He added that Il Duce “believes the democracies will still give in.”^29^ The decision to obliterate Poland was therefore fixed before the pact with Russia was signed. Without it the Nazi Panzer divisions would have rolled up to the borders of the Soviet Union, occupying the White Russian and Ukrainian half of Poland to which the Soviet Union had a far better right.
This fact alone should dispose of the contention that if the Soviet Union could not come to terms with Britain and France it should have at least stood neutral like the American Congress. Moscow, it is said, did not need to make a deal with Hitler and give him the green light, but in reality the Soviet Government did not have this choice. By standing aloof it would have lost not only Eastern Poland but the Baltic states as well. By rejecting Hitler’s promises, and the threats that always went with them, the Soviets would have placed themselves in the daily and imminent danger of fighting the German-Russian war which they believed the West had tried to bring about.
By making the truce with Hitler the Soviets gained four things. (1) They got everything in the Baltic states which the Allies had refused them, and more, plus the ability to ship home to Germany 100,000 Baltic Germans, as well as 300,000 other Germans from Poland and other Eastern areas. These huge fifth columns were quickly cleaned out of the Russian sphere, to the deep chagrin of the Nazi supermen. (2) They achieved freedom to correct their boundary with Finland and reclaim Bessarabia from Rumania. (3) Instead of incurring the full power of the Nazi war machine, while the West viewed their plight with satisfaction, they turned Hitler back upon the West. (4) They also acquired nearly two years of precious time in which to prepare for a German onslaught.