Kwantung Army was a big group of soldiers who belonged to Japan. They lived in a place called Kwantung which is in China. Just like you have teachers and parents who take care of you, the Kwantung army had a boss who was in charge of all of them.
The Kwantung army was sent to China by Japan to control the area, but they didn't always treat the people there very nicely. They would do things that were not fair, like taking things that didn't belong to them or hurting people who didn't deserve it.
During a big war a long time ago, the Kwantung army did some really bad things. They attacked a place called Manchuria and took it over. They weren't supposed to do that and it made a lot of people mad.
Even though the Kwantung army did some wrong things, they also helped Japan in some battles. However, in the end, they were punished for the bad things they did and had to give up the land they took over.