Japan was already making moves in what was then Manchuria. They even assassinated the warlord they were funding once they felt he was no longer useful and wanted to replace him with a more malleable puppet.

That warlord’s son Zhang Xueliang is a controversial figure, but he did play a major role in arresting and forcing Jiang Jieshi to postpone his civil war and focus on the invading Japanese.

Guess it wasn’t so much that they were able to defend against Japan, it sounded more like Japan was preparing and planning their invasion into Manchuria and then into the rest of China.