The USSR tried making pacts with the West first, because they 100% expected an attack from the Nazis at some point. However, Europe refused. Remember, the Russian revolution happened during WWI along with years of civil war after, a lot of it encouraged and pushed by the same Western countries they were now trying to make pacts with, countries who funded the Whites during the Civil War or would straight up invade them. These countries did not like communism and were probably annoyed they lost the civil war. But, they were exhausted by war and ended up having to acknowledge the USSR as a country after awhile. But it didn't mean they had to make a pact with them, and it did probably mean they secretly hoped the Nazis attacked the USSR first. As a result, these countries hoped to do the same thing dipshits like to blame the USSR for, making a pact with the Nazis so they'd attack the other side first and they could avoid a war.

This left them with the same problem. The USSR knew they weren't ready with a war with the Nazis, or anyone, and needed to rapidly develop their industrial base. Originally they were hoping they could catch up industrially and technologically through the West, but when Germany didn't have their communist revolution, the truth began to rear it's ugly head. They were alone in the world, the only communists, and hated by every other capitalist country.

Meanwhile, in the West, like I said before, they were excluding the USSR from their own peace agreements and trying to make ones with the Nazis instead. The first of these agreements was famously called the Munich Agreement. When the Germany started probing Czechoslovakia, the USSR offered to come to their aid if France did, too, and Poland and Hungary Romania let them through. Everyone refused. Instead, Nazi Germany, the UK, France, and Italy met up, purposefully excluding the USSR. That's the agreement where they ceded Czechoslovakia to the Germans. They basically did their own Molotov-Ribbentrop and separated parts of Czechoslovakia territory to the Germans, Poland, and Hungary, to, similar to the USSR, avoid war. But for some reason, the USSR are called Nazi allies for this, while the West is just small, naive beans for trying to take Hitler's side and hoping they'd attack the USSR, who is being left out in the cold during negotiations.

Abandoned by the West, they were forced to make a non-aggression pact with Germany themselves. Both sides knew it wouldn't last and war was coming eventually. Hitler wasn't exactly secret about hating communists in his speeches and Stalin wasn't secret about how they need to defend themselves from the fascists, so the USSR used the opportunity to develop an industrial base as fast as possible. They also moved forward and took some of Poland (with Germany taking the other half) as a buffer zone for the eventual attack, too, and as a side result saving half that country from the Nazis (well, for a period of time).

The rapid industrialization led to some terrible working conditions and many of the other horrible downsides of capitalism coming back to the USSR. Many times, the "outside enemy" is a good excuse for abusing and exploiting your people, to make them scared and offer safety. But on the other hand, in this case, he was right. And he had a good reason to be - they were just invaded by numerous countries during a Civil War a few years before, the West refused a pact, Germany just purged all their communists and were becoming more fascist, militarized, and anti-communist by the minute, and the USSR was still behind, and weak and vulnerable.

So in the end, of course they were attacked. Despite the preparations, Stalin was caught off guard for various reasons, I suspect the biggest being the timing. He probably didn't think Hitler would be dumb enough to start a second front before finishing off the British, hoping for another year or so, but who knows. Although it seems very stupid on Hitler's part, and obviously with hindsight it lost them the war, it makes sense with that logic - he wanted to betray the communists before they were fully prepared and when they least expected it, and before their industrial base caught up to the West.

Last resort by the Soviets while rabidly developing and preparing for war. They knew the pact would not last.

Edit: https://hexbear.net/comment/7518080 the other reply is much better.