The upcoming leader of country A gets killed by a terrorist.
Country A demands country S to extradite members of that terrorist group.
Country R opposes this, followed by country F and B.
In the meantime, country G has backed A with support and troops to extinguish the terror, coming from S.
B F and R threat G, if it continues to support A, there will be retaliation.
G does not back down, instead gives A full support.
A launches an attack against S to extinguish the terror threat.
G sends troops, too.
B F and R declare war on A, G now helps A and declares war on all 3.
That is your basic WW1 story.
Written down in that way, it looks so different today, what is currently celebrated in Somme and Verdun.