Why did the war start?
Since you have not specified which war you are asking about, it is hard to answer this question. I will address general reasons why wars start and relate those reasons to World Wars I and II.
Wars generally start because one country or another is not satisfied with the level of economic, political, or military power that it has. That country then tries to gain more power. When this happens, other countries feel threatened because they are afraid of what the country’s efforts will do to their own levels of power. This often leads to the outbreak of war.
Let us look at how this worked in the two great wars of the 20th century. In both cases, Germany, along with some other countries, was not happy with its status. Germany felt that it deserved a more prominent position in the world before WWI. After WWI, it was angry at the blows to its prestige that were inflicted by the Treaty of Versailles. In both cases, Germany worked to increase its military and political power.
In both cases, countries that were stronger than Germany or were about the same strength became worried. They worried that the rise of Germany’s power would be a danger to them. In both cases, they eventually went to war to prevent Germany from increasing its power too much.