Well, Japan needed resources and we wouldnt really trade oil with them, Japanese didnt want US to join the war so they felt if they attacked then we wouldnt really fight back. Also, the Japanese felt they could beat the US. But you have to remember, there was a depression world wide and Japan needed their resources.

the external factors led to Mails decline is that after Mansa death, the descendants argued about who should be the next ruler of Mali ,the raided and burned it.