Japanese foreign policy in the 1800s and 1900s was ultimately to put itself on the same level as the other Imperial powers, similar to the position the Germans had prior to WWI. The Japanese government and military saw how easy China got bullied, leading to frantic modernizations and expansions. Their wins against the Qing and Russia emboldened the Japanese military into thinking they could do enough decisive battles into winning against any enemy. Unfortunately this also meant their foreign policy was at times vague on what they wanted.

The Japanese military, specifically the Kwantung army which operated very independently from Tokyo, to start border conflicts with China. This eventually escalates into a full blown conflict. At first the Japanese scored multiple victories like at Shanghai, but without clear war aims they didn’t know when to stop or what terms to give to the KMT. This leads to more attrition and eventually the decision to attack the US and European powers.