Good grief… if the U.S.S.R. was the Reich’s ‘ally’ then so was Poland, France, the United Kingdom, and every other piss bucket in Europe.

To quote my thesis:

It was no doubt disgraceful that Soviet Russia should make any agreement with the leading Fascist state; but this reproach came ill from the statesmen who went to Munich. […] [The German–Soviet] pact contained none of the fulsome expressions of friendship which Chamberlain had put into the Anglo–German declaration on the day after the Munich conference.

Indeed Stalin rejected any such expressions: “the Soviet Government could not suddenly present to the public German–Soviet assurances of friendship after they had been covered with buckets of filth by the [Fascist] Government for six years.” The pact was neither an alliance nor an agreement for the partition of Poland. Munich had been a true alliance for partition: the British and French dictated partition to the Czechs. The Soviet government undertook no such action against the Poles.

They merely promised to remain neutral, which is what the Poles had always asked them to do and which Western policy implied also. More than this, the agreement was in the last resort anti‐German: it limited the German advance eastwards in case of war, as Winston Churchill emphasized. […] [With the pact, the Soviets hoped to ward] off what they had most dreaded—a united capitalist attack on Soviet Russia. […] It is difficult to see what other course Soviet Russia could have followed.

— A.J.P. Taylor, The Origins of the Second World War

The Kremlin wasn’t staffed with amnesiacs; they had the common sense to know that the capitalists were going to reinvade Soviet Eurasia. The question was when; intelligence reports were often contradictory, which was why Moscow hesitated before fighting back.

See Molotov’s explanation.

[I]t was impossible not to miscalculate. How could you know when the enemy would attack? We knew we would have to deal with him, but on what day or even what month. […] We are blamed because we ignored our intelligence. Yes, they warned us. But if we had heeded them, had given Hitler the slightest excuse, he would have attacked us earlier.

We knew the war was coming soon, that we were weaker than Germany, that we would have to retreat. The question was, retreat to where—to Smolensk or to Moscow, that’s what we discussed before the war.

We knew we would have to retreat, and we needed as much territory as possible. We did everything to postpone the war. And we succeeded—for a year and ten months. We wished it could have been longer, of course. Stalin reckoned before the war that only in 1943 would we be able to meet the Germans as equals.

[…]

On the whole, everyone expected the war would come and it would be difficult, impossible for us to avoid it. We delayed it for a year, for a year and a half. If Hitler had attacked us half a year earlier, you know, bearing in mind our situation then, it would have been very dangerous.

So it was impossible to begin obvious preparations without revealing to German intelligence that we were planning serious measures. We took many serious steps, but still not enough. We didn’t have time to finish very much. Some think Stalin should have to answer for all this. But there was the people’s commissar for defense, the chief of the general staff…

(Source.)

Also, that Wikipedia link does not support the Redditor’s claim.