The effects of the war included 8.5 million deaths just from soldiers, 16.1 million in total, higher taxes, rationing of food and other products, a propaganda war, and famine. Germany was blamed for the war so they owed millions of dollars in reparations to many different countries so Germany had to skyrocket their taxes and began a famine.
Personally, I believe that World War I could have been prevented if it weren't for certain actions. For example, if Austria-Hungary had negotiated seriously with Serbia in July 1914, war could have been avoided. However, Germany egged Austria-Hungary on to be completely unreasonable. It turned out to be the result of a hundred little pressures all building up until eventually it just exploded. Also, if it weren't for the three alliances it could have possibly been avoided.
I would say that World War I was justified in the fact that Germany was trying to take over Europe in a very bad and hostile way so they had to be stopped. It's hard to say that a war that caused millions of deaths was justice but when you think about it, they had no right to take over Europe and would've hurt and killed many innocent civilians in the process. Germany ended up paying their debt to the war by owing reparations and money so in another sense it was justified.