May 11, 1945
The timing of the invasion was just about right. The Poles had chosen to attack in spring, giving them time to complete the conquest of Japan before winter set in and made things harder.
Although Japan as a whole was well-defended, there were bound to be weak spots. One weak link was the northern tip of Honshu. Another was the island of Kyushu. It was decided to secure Kyushu first. While the Japanese home army seemed formidable on paper (and still was) much of it was composed of garrisons and militias. The garrisons could not be moved around easily, and would immediately surrender or be annihilated upon defeat.
Japan's defenses were formidable, as these reconnaissance flights showed.
The marines were put on offensive supply for what might be the largest amphibious campaign of the war. The invasion fleet sailed from Pusan and took up positions off Kyushu. The southern half of the island would be the target. As day broke, six divisions of Polish marines landed and began their assault. This would be their first live combat situation. Opposing them were three garrison divisions, a far lighter defense than elsewhere in Japan, but not as light as could be hoped. Because the invasion force was relatively small, it would need all the support it could get from the navy and air force.
Seven hours later, the Japanese garrison divisions were close to breaking. Polish bombers also moved in to speed up their defeat.
Interestingly, note that the French had occupied the Amami Islands northeast of Okinawa, and were now attacking the Japanese defenders at Okinawa. These attacks would fail however. Despite Okinawa being defended by just one division, it would repel all the French attacks. The French should have used marines.
The Japanese were eliminated and the marines landed on the island. After taking some time to organize themselves from the landings, they turned their attention to the northern half of Kyushu. Fukuoka turned out to be defended by one garrison and one infantry division. With this victory, the Poles now occupied the whole of Kyushu. A corps of three infantry divisions was ferried from across the Tsushima Strait to keep Kyushu from being retaken by the Japanese in Hiroshima.
Taking Kyushu had been fairly easy, but now the real hard part began. Now the whole invasion force (besides the three divisions in Kyushu) was taken to off the Pacific Tohoku coast. Sendai was also discovered to be fairly lightly defended. The marines would assault the beaches, and the rest of the troops would land once a beachhead was established. The attack began at daybreak of May 21, 1945.
Again, the area was defended by three Japanese divisions. After another tough fight, these divisions were defeated. Two were garrisons, but one was an elite Japanese marine division. This division was now retreating from the battlefield. A lone retreating division was the perfect target for aircraft, so Polish bombers were sent in from their bases in Korea to bomb it into oblivion. Destruction of special divisions such as marines was a priority.
Poland's fighter wings were also conducting air superiority missions over Japan, along with many other Allied aircraft, because the Japanese did still have an air force. Losing a transport flotilla loaded with troops to the Japanese would be a severe loss. Fortunately, the patrols succeeded at catching Japan's fleet of naval bombers in the air over Hiroshima.
Note that British, American, and Polish fighters all took part in this battle.
These interceptions would continue during the course of the invasion, keeping the Japanese from using their air force.
Soon after that, the Marines landed in Sendai, followed soon after by the rest of the invasion force. The Poles now had a foothold on Japan.
Note the large number of troops in Tokyo, these would move north to Fukushima.
This didn't mean things were going to get any easier, though. The Japanese had huge numbers of troops on Honshu, especially around the Kanto and Kansai regions. However, the swift capture of Kyushu and landing in Tohoku had caused the Japanese to begin moving their troops all over Japan. The Poles could take advantage of this confusion to win against much greater numbers...
The difficulties of the campaign were revealed when the Poles attempted to expand their area of control. Fukushima was undefended when the Poles landed, but was quickly reached by a large number of Japanese troops from Tokyo, forcing the Poles to stop their attack. Next, the Poles attacked mountainous Akita with Sosabowski's mountaineers, supported by infantry. The goal was to cut off the defenders of Hokkaido, and provide a second axis of attack on Fukushima. This was important as the Poles made an oversight in planning the invasion - they failed to bring a headquarters division along. With no field marshals in the attack force, nine divisions was the maximum that could attack from one province without suffering crippling penalties.
Though they defeated the Japanese in Akita, once the Poles occupied the province, they themselves were attacked by a portion of the large Japanese stack in Fukushima. They were forced to retreat.
On the plus side, during this time a Japanese HQ division was caught alone and unentrenched. This valuable target was quickly destroyed by Polish bombers.
With the large group of Japanese troops in Fukushima making matters risky, it was noticed that the haphazard movements of Japanese troops all over Japan had left Hokkaido relatively undefended. A portion of the invasion force was detached to take Hokkaido. In fact, the northern half of Hokkaido was completely undefended, and this is where the Poles landed. However, six garrison divisions were defending Sapporo, Hokkaido's largest city. The Poles attacked them with nine infantry divisions, again supported by shore bombardment and air support. The Japanese would be unable to retreat because Polish ships blocked the Tsugaru Strait, and in any case Akita was still in Polish hands, though not occupied by any Polish troops.
Since Poland had never been able to use its carriers in fleet-to-fleet battles, shore bombardment was pretty much the most useful function for them.
The six divisions were annihilated and Hokkaido secured entirely. Now, both the northern and southern of the Japanese Home Islands were in Polish hands, including much of Japan's resource production. It only remained to take Honshu and Shikoku. The Poles, having fewer forces than the Japanese, were adopting a strategy of not landing in one place and proceeding inland from there, but instead being maneuverable and striking across the least-defended coastal provinces of Japan. This took advantage of the fact that out of Japan's total strength on paper, around 40 divisions were stationary garrisons that could not move on their own and could be eliminated when defeated. Despite this strategy of strung-out attacks however, the Poles would still have to fight the large Japanese armies eventually.