The First World War was declared on the grounds of contradictions, which existed between the leading European countries. First of all, after the Franco-Prussian War, the united German Empire became the leading European country. As a new dynamic European empire, Germany aimed to become a political, economic, and military leader on the continent. Apart from this, it pretended to equal rights in the colonial dominion of Britain, France, Portugal, Belgium, and carried on a fierce struggle for sales markets.
Secondly, France aimed to return Alsace and Lorraine, which were separated from it by Germany after the Franco-Prussian War in 1870, and aimed to preserve its colonies, especially South Africa, at any price. Besides, French goods could not compete with German ones, which significantly weakened its economic force.
Thirdly, Russia insisted on its exclusive right to take a control over all Slavonic nations and stood up against German and Austrian invasion of the Balkans. Thus, the three reasons for WWI were striving for political and economic influence as well as for exclusive domination over other nations.
It seems to me that all the three reasons mentioned are valid reasons for a war. It is common knowledge that the struggle for political power always existed, and powerful people were ready to stake on everything to gain the power and control over other countries and nations. The same can be said about striving for exclusive economic dominance since the latter is the basis for the welfare of any country. As colonies ensured quick enrichment of metropolitan countries (since the former were the primary sources of raw materials, which were bought at lower prices and sold at higher ones), there is no wonder that the struggle for them was inevitable. Nevertheless, the wars based on such and alike reasons are inhumane since they are nothing but an aggressive encroachment upon liberty and normal development of other countries.