For starters, the Japanese military was built around winning a conflict quickly and decisively. Their personel, especially the pilots of their revered air-forces were highly trained and thus could not be replaced very quickly. Things didn’t fair well when the Japanese were pulled into a long term conflict in the pacific, rather than a short and decisive victory against the US like they planned. By 1945 when the Soviet’s invaded, the Japanese military was effectively crippled.

Now, let’s analyze the situation between the two states. As it is very important for this topic.

Prior to the Soviet invasion of Manchuria, the two states shared a neutrality pact. Freeing up troops on both sides for fighting on other fronts. Because of this, the Japanese saw no need for a military build up in this area. Once, the Germans were near capitulation, Stalin expressed wishes to make gains in the Far East. The Soviet’s decided they did not want to continue the neutrality pact and informed the Japanese of this in April of 1945. Although, assuring them that their would be another 12 months before it fully expired and they had nothing to worry about. In May, the German surrendered leaving only Japan in the war against the allies. The Japanese began attempts to enlist the Soviet’s in peace talks with the other allies and to remain at peace. The Soviet’s went along with this and pretty much drew out the process as much as possible. Giving the Japanese false hopes.

In August of 1945, the Soviet’s invaded Manchuria. Breaking the neutrality pact. Catching the Japanese by surprise. They came with a force twice the size of the Japanese forces stationed in Manchuria. Using a massive pincer movement, aimed at enveloping the Japanese forces. The Japanese army in Manchuria had been whittled away due to troop redeployment to the pacific. The equipment present was outdated as well. This paired with mistaken assumptions of where they would be attacked from. Meant they weren’t in any position to put up a fight.

As you can see, problems for the Japanese arose far before the invasion even happened.

I hope this wasn’t too much to read. And I hope my writing was adequate, as my English writing isn’t the best.

I would like to add a couple of points to what u/ItsMeTheArab said in his comment.

First of all, the Kwantung Army of 1945 was not the same army as it was in 1937. In 1937, the Kwantung Army was the pride of the Imperial Japanese Army (IJA). It had the best officer corps, the best equipment and the most experienced divisions. By 1945, the war had long since moved on. Understandably, officers, men and equipment were stripped away to aid in the prosecution of the war while Manchuria became a backwater. It was still large and fairly competent in 1945 but also short on equipment and no longer inspired the respect the name once deserved.

Second, we need to understand what happened in Manchuria between the Soviets and the Kwantung Army. The majority of the war occurred between August 9th, 1945 when the Soviets declared war, and August 16th when the surrender of Japan was communicated to the IJA in Manchuria. To describe the actual movement and combat I will use a combination of quotes and paraphrasing from Tsuyoshi Hasegawa's book, Racing the Enemy (pg. 195)

>The Soviets would make their moves from three directions: The Transbaikal Army would come from the west and the First Far Eastern Army from the east, while the Second Far Eastern Army would thrust from the north, all in the direction of Changhun and Mukden.

Soviet forces numbered 1.5 million men, while the Kwantung army had 713,000 with another 280,000 men spread across Korea, Sakhalin and the Kuril Islands. The Transbaikal Army met no resistance and was only slowed by the Gobi desert, a lack of fuel, and the Great Khingan range.

The Eastern thrust met heavy resistance in Mutanchiang where they were engaged in house to house fighting until the Japanese surrendered on the 16th. The Soviet army had travelled unopposed approximately 250km into Manchuria before reaching Mutanchiang. A second thrust was made in the East into Korea where Soviet forces managed to take Chongjin also on August 16th when the Japanese surrendered.

It's important to point out that while the majority of Japanese forces surrendered beginning on the 16th, not all did. The relationship between the IJA, the Japanese cabinet, and the Emperor was incredibly complicated from western standards leading to several military coups during the Showa period as well as several IJA units refusing the Emperor's command to surrender. The truth of the matter, though is that the majority of steamrolling happened against an outnumbered, inferior enemy that was surrendering en masse.