In the year 1935, almost all of Africa was under colonialism under Britain, Spain, France, Portugal, etc. How would they muster an army to the defense of Ethiopia? I learned from a documentary I watched that the Italian invasion of Ethiopia was consented to by all these European colonialists. So they turned a blind eye when Italy invaded Ethiopia.
You know what? Ethiopia, at that time, was the only African country left standing uncolonized. Hence, these European colonizers and colonialists backed one of their own, Italy, to invade Ethiopia and subject them also to colonialism like the rest of Africa.