Jerusalem is a major landmark in all three religions
It was a failure because they had lost pretty much everything
Nationalism heightened in the 19th century and heading into the 20th century. The nationalistic fervor by people in Europe had them each viewing their own nations as better than the others, in competition with the others. This would lead to an increase in tension between the nations.
Imperialism expanded on that nationalistic rivalry by carrying their competition to other parts of the globe. The nations of Europe sought to grab control for themselves over parts of Asia and Africa. When war erupted, that also meant it would become a world war because the European nations would include people from their imperial territories in the war.
Militarism grew ever more potent as the 20th century opened. The competition between nations included a massive arms race in terms of expanding armies and navies. The nations also sided up in competing military alliances and made military battle plans as to how they might fight a war if war came. When a cause for war broke out, all those preparations propelled the nations of Europe into war recklessly.
