“What led to WWI?, etc…”
The chet of the meraglim (you cried for nothing, I will give you something to cry about), which delayed going into e’y, which ultimately led to moshe rabbeinu not leading the bnei yisroel into e’y, eventually made its way to the churban bayis rishon and sheini, this long galus and all its trials and tribulations. When did the bnei yisroel cry for no reason? The night of tisha bav. So, one can legitimatelystate that everything IS rooted in tisha bav.