I add an answer because I want to shade some points of the accepted answer.

First point: Soviet and Stalin's point of view on the conflict.

The accepted answer gives anachronic vision of the situation. Stalin had no clear vision of the army vs navy conflict in Japan about strategic directions where to expand. Elements supporting that point is that he never trusted his famous spy Richard Sorge when he said that the Japanese had abandoned the idea of a Siberian conquest.

Instead of a strategic calculation about Japanese internal politics, the Soviet reaction to the conflict was a simple "You shall not pass" politic: The Japanese view of Siberia were seen as the following part of the 1904-1905 war, and Stalin, for "prestige" issues (which are directly linked to being overthrown in such a regime), could not support a defeat nor any territory loss.

Second point: The fact that the conflict was not important for Japan

That is true in the idea that the Japanese army had the tendency to start conflict "on its own". But overall, the issue was not a low politic willingness to fight rather than a lack of logistic preparation and fightings in China that hampered efforts.

Third point: Barbarossa being a failure

I agree with the overall ascertainment, but this is not an argument: the targets of Barbarossa and Khalkin Gol were not of the same scale, and this is exactly the inability of the Germans to stop when they were successful that led them to disaster (+ the two fronts fighting). The Japanese, achieving in Siberia (a less developed territory so less important for USSR) a quarter of what Barbarossa achieved, would have been satisfied and started negociations.

On the other hand, I admit that there were Japanese "they will not come back" thoughts after the initial fighting in Khalkin Gol, and that this contributed to the difference of forces between Soviet and Japanese armies during the 2nd phase of Soviet counter-offensive.

Other points:

I entirely endorse the other points: difference of context for Winter War and Barbarossa, good generals being still there in the Red Army and sent at Khalkin GOl (notably Jukov).

This remains even today a Battle very much shrouded in mystery...and it's not for lack of Historians digging into the archives.

The only substantive reading on the Battle that I have read is that there were 3 Russian Commanders including Zhukov with one of the other two being executed.

According to most historical accounts this was the first attempt by the Red Army to put into effect what later became known as "Deep Battle Techniques" which involved massive encirclements at great distance to one another and from the enemy itself. For obvious reasons this was impossible to do against Finland...although apparently not to the Red Army at that time...but Historians seem to agree this technique is what devastated the German Armies starting in Stalingrad and then going forward.

The significance of the Battle itself is that it brings for the first time in World War 2 the name "Zhukov" to the attention of the PTB imho.

Japan and Stalin's "newly created USSR" (Stalin was Georgian and never made any attempt to hide this fact) had a non aggression pact...which in the waning days of WW 2 was reneged upon unleashing a truly massive Army upon Soviet Russia's Eastern frontiers. Japan's unconditional surrender quickly followed soon thereafter very much giving us the World we have today.

First up, the effect of the purges is largely overplayed. There were many factors that caused the red army to fail so spectacular during Barbarossa. One of them was a massive mobilization effort that put many units in a state were they were de facto useless due to a lack in artillery, ammunition, fuel and even boots. Not to mention trained personal. The units facing the Japanese in 1939 weren't the best units to begin with… but at least they had all their equipment and the men they needed. And there was quite a lot of them.

On top of that the BT tanks are pretty good tanks. Especially for the rapid advances Zhukov ordered during the counteroffensive. Japanese infantry and artillery had very little training in anti tank roles. And while Japanese tanks could destroy them with relative ease, so could the soviet tanks with the Japanese once.

And then of course the fact that all the advantages the Soviets had were used during the counter attack. Primarily superior numbers. A force twice as large with fast units that can pull of an encirclement and require your own tanks to be redirected to deal with. That's hard to counter if you have been shooting at small groups of unorganized soviet soldiers. The Japanese probably didn't expected that there would be a reaction with such intensity... ever.

Or in other words, Zhukov had a sledgehammer the whole time. And after months of being tickled by the Japanese he finally swung it out of nowhere.

The other answers are fantastic, but one more one important point: The perception that the "the USSR was very weak". This perception could be the result of anti-Russian and anti-Soviet propaganda, and is not well grounded in a factual analysis.

Stereotypes about Russian bureaucracy and culture are widely present in historical English language media. Another example of this is the assessment of the 1905 Russo-Japanese War. I have often heard some horrible assessment, such as "If the Russian weren't so inefficient, the (non-European) Japanese wouldn't have stood a chance." Meanwhile, military historians have spoken about the high quality of both Russian and Japanese forces in this war, and pointed to this conflict as setting the stage, tactically speaking, for WWI, and suggested that supply chain length/ bad choices by Tsar Nicholas II were primary reason that Russian troops didn't win.Source Here Certainly British / American interests in opposing the southward expansion of Russia in the great game didn't help Russia in the final outcome in 1905.

Short answer: Soviet equipment was quite good, but their commanders stank. They got lucky that a few brilliant commanders survived the purges and were given a free hand at Khalkhin Gol.

The problems of the Soviet army in WWII are more ones of leadership than material. They had the men, the equipment, and the tactics, but they had few good leaders to take advantage of them.

Prior to the purges, the Soviet military was feared. Their military was enormous, equipment the interwar standard range of awful to revolutionary, and their tactics excellent. In the 30s, Soviet doctrine followed Deep Battle which is remarkably similar to Blitzkrieg.

Then the purges happened. Most of the top level leadership was killed or banished to obscure posts. Those who remained or were promoted knew more about political reliability than military tactics. Yet pieces still remained of the interwar brilliance.

One of these pieces were the Soviet leaders who were brought in to escalate and win the growing border skirmish, one was Georgy Zhukov who would survive all the purges and go on to win the war for the Soviets. The Soviets built up their strength (500 tanks and 800 aircraft to the Japanese 135 tanks and 250 aircraft), defended against attacks, and then executed a classic double envelopment using concentrated armor to break into the Japanese rear area.

In contrast The Winter War with Finland was planned and fought by leaders who showed little imagination than to bludgeon the Finns. The Soviets relied on frontal assaults, used poorly trained troops, and brought tons of useless anti-tank equipment which fell into the Finns' hands. It was only after a pause and shakeup of leadership and tactics did the Soviets made headway. However, they did show a logistical brilliance in their ability to move and supply 21 divisions (far more than the Finns expected possible) all across the inhospitable Finnish border. The Soviets won their objectives, but the Finns also won in keeping their nation intact. Both took horrific casualties.

Khalkhin Gol was a success, and so would not have made the Soviet high command reflect. It was the humiliation of the Winter War which caused the Soviets to begin reforms, but it didn't happen fast enough to be ready for the German invasion.

The Japanese were totally unprepared for modern tanks. They had no antitank capability and little to no practice with fighting tanks. When they got into a battle in good tank country with a mess of Soviet tanks, the result was predictable.

The Finland failures were due to the inability of the Soviets to deploy and fight their forces in the rough country and winter weather of Finland, not that they couldn't fight. And in the end, the USSR won that war.

The German army was just superior at armored warfare in Barbarossa.

Neither situation applied to Khalkhin Gol - the Soviets could deploy superior forces and thrash the Japanese.

The Soviets and Japanese were operating in two different worlds. So the respective "characterizations" were relative to their worlds, not compared to each other.

The Soviets suffered mainly in comparison to the German forces, which were the best trained and led in the world. Even the Finnish forces they faced in the Winter War were (indirectly) German-trained (under Mannerheim).

The Japanese distinguished themselves in combat against poorly armed Chinese, badly trained "colonial" (not elite) British and Dutch forces, and unprepared American forces. This was true even for Japanese units equipped with World War I vintage weapons.

But when they met, the modern, Europeanized Soviet forces beat the backward, Asiatic Japanese forces. Specifically, the Soviets had good tanks that the Japanese couldn't counter. And Marshal Zhukov was the most able Soviet commander, who found himself in the Far East for "political" reasons.