How did the Soviet Union Beat Japan so fast in WW2?

There were a few main reasons for why the Soviets were able to succeed so dramatically during the Manchurian Strategic Offensive Operation (sometimes known as Operation August Storm), when the Japanese were known for fanatical resistance: 1) the Kwantung Army of 1945 was a shadow of its former self, stripped of its best troops and equipment; 2) the terrain in Manchuria was far more conducive to offensive operations than the various Pacific Islands and the Japanese were ill equipped to resist Soviet style offensive operations in the best of times (and the Japanese still resisted intensely where they could); 3) the attack itself came as a surprise, with almost all remaining Japanese resources focused on facing the anticipated American landings on the Japanese Home Islands.

To start with the first point, the Kwantung Army was the main Japanese army group in the puppet state of Manchukuo. In the past, it had been a formidable force, as the Imperial Japanese Army (IJA) anticipated war with the Soviet Union and had dedicated substantial resources to building up the Kwantung Army in anticipation of that conflict. But the outbreak of the Second Sino-Japanese War in 1937 had drawn Japanese forces into the long, protracted war in China. While the Kwantung Army had been expanded up to 16 divisions during the Kwantung Army Special Maneuvers (Kantokuen) in July of 1941 to potentially renege on the Soviet-Japanese Non-Aggression Pact and take advantage of a potential Soviet collapse, it was steadily drawn down over the course of the war as the continuing demands of the war in China and the Pacific reduced its importance. By January of 1945, the IJA was actively drawing down its forces in Manchuria, bringing the best divisions back to the Home Islands for the anticipated Decisive Battle there, before Allied interdiction campaigns severed even the shipping lanes between Japan and its closest imperial possessions. Eleven front line divisions were pulled out of Manchuria for the defense of Japan itself, and--with the fall of Okinawa in June 1945, the the General Umezu Yoshijirō the Army Chief of Staff notified both the Kwantung Army and the China Expeditionary Army that they could expect no reinforcements or assistance from Japan, as preparations for the defense of the Home Islands would take top priority of whatever limited resources were left. The Kwantung Army sought to rebuild its strength by calling up reservists from across Manchukuo, ultimately raising nearly 700,000 troops. However, these were not the equal of the divisions which had been recalled to Japan and were ill equipped.

Onto the second point, Manchuria was very different terrain from the small Pacific islands where most of the infamous battles between the Japanese and Americans had taken place. While most of the Pacific island battles had taken place in terrain with significant rough terrain, which the Japanese could honeycomb with tunnels and defensive fighting positions, much of the Soviet-Japanese frontier was relatively open, flat, land. Moreover, on the tight Pacific islands, there was no room for the kind of vast sweeping armored offensives of the kind the Soviets launched into Manchuria. Similarly, there was very little room to retreat, and so the Japanese would simply have to defend their fixed positions--often to the death--while the Americans would have to try and extricate the Japanese from their extensive fortifications, often by close quarters assault. Manchuria was simply too big for this kind of operation and the Soviet offensive was able to quickly outmaneuver Japanese units, which had always been short on motorised or mechanised transport. As far back as the Nomonhan Incident (also known as the Battles of Khalkin Gol) in 1939, the Japanese had been aware of their limitations in countering Soviet material superiority in armor, artillery, and logistics, and the gap between the Red Army and IJA had only grown in the 6 years of war between the two clashes. This is not to say that Soviet victory was total of course. Indeed, Soviet preparations for the attack had been rushed and pushed ahead of schedule in order to ensure that the Soviets entered the war against Japan before Japan surrendered to the United States. The Transbaikal Front, advancing into Manchuria from the west was slowed by substantial logistical constraints even without significant Japanese resistance had to create advance detachments to concentrated limited fuel in order to continue advancing. Kwantung Army units resisted fiercely in Korea and eastern Manchuria, where urban fighting in Mutanchiang would last until August 16. Yet, despite this fierce resistance Soviet forces were able to bypass these Japanese strong points and continue their advance. As a note on timescale, the Shōwa Emperor had made the announcement of Japan's surrender on August 15 and on August 16, Kwantung Army HQ issued orders that all of its subordinate units were to cease hostilities and surrender to Soviet forces. At this point, the Soviets were now advancing without much organised resistance, but on August 15 the Soviets were still far away from the major cities of Harbin, Mukden, Changchun, and Kirin, let alone the prizes the Soviets wanted most: Port Arthur and Dairen on the Liaotung Peninsula. The negotiations between Chiang, Stalin, and Truman could be an answer on their own, but suffice it to say that the Soviets were looking to the post-war, and it was imperative that the Red Army's tanks reached Dairen before the U.S. Marines.

Finally, the Soviet offensive against the Japanese came as an operational surprise. Japanese military intelligence had been warning of a Soviet military build up in the Far East but Japanese political leadership had been heavily invested in peace feelers to Moscow, hoping to have Stalin broker a peace between Japan and the West that would come short of unconditional surrender. In fact, when Ambassador Sato arrived at Soviet Foreign Minister Molotov's office on August 8, he was arriving to present a proposal from the Japanese government to have former Prime Minister Prince Konoe come to Moscow in a continued effort to arrange a negotiated settlement. Instead, what Sato received was the Soviet declaration of war. Confusion was rampant in both Tokyo and at Kwantung Army HQ in Changchun. In Tokyo, political leadership was shocked by the news of the Soviet invasion and declaration of war, as the entry of the Soviet Union into the war was the worst case scenario they had been striving to avoid at all costs. Meanwhile, the Kwantung Army's plans for the defense of Manchukuo had always been a series of staged withdrawals with a final goal of holding onto defensive positions in northern Korea. Within a few hours of the Soviet invasion, the Kwantung Army had already decided to abandon the Manchurian capital and begin its retreat south.

To summarise, Soviet success in Manchuria relied on a number of factors. Their opponents had not only been severely weakened but also neither expected their attack, nor were they even overly concerned with holding onto the territory the Soviets were advancing into. Even so, it's not as if the Japanese simply offered no resistance to the Soviet attack. Fierce fighting occurred in several areas, but the terrain of Manchuria enabled the Soviets to simply bypass these strong points and continue on. Moreover, active resistance only really lasted a week, with Soviet troops crossing the border on August 9 and Kwantung Army ordering an end to hostilities on the 16th. For the remainder of that time, while substantial confusion reigned over the precise status of Japanese forces, Soviet forces continued to advance and did not reach many of their key objectives until the 20th or even later.

I hope this has helped answer your question, and please feel free to ask any follow ups.

Sources

Edward J. Drea, Japan's Imperial Army: Its Rise and Fall, 1853-1945
Tsuyoshi Hasegawa, Racing the Enemy: Stalin, Truman, and the Surrender of Japan