Alliances- A system to bring peace, however it brought war. The upsides was that peace was able to be expressed within those who had an alliance, however alliances would support each other and this could lead to the support for each other to motivate war. Such as the Axis powers which was formed by Japan, Germany and Italy because of the growing strength they had of expanding and creating a colonial empire, such as Mussolini expanding into Africa. 
Imperialism- process of expanding a country's territory by using military power. For example Japan expanding their country by invading Manchuria which had a heavy amount of coal and iron which lead to the invasion of North China. Additionally, Mussolini lead the invasion of Ethiopia in order to expand Italy's colonial empire in Africa.

Nationalism- a patriotic feeling which grows the interests of a nation. Nationalism had the potential of making a nation together however can also increase the possibility of competition of nations initiating in order to see which overpowers which. Nationalism played a factor in the war because there was economic problems which needed to be solved with facing the needs of the nation such as Japan invading Manchuria in order to build mines and factories to provide goods to Japan. As well as Germany breaking away from the league of nation's orders and seizing territory instead to expand territory.

Assassination of Archduke Franz Ferdinand of Austria angered the Austro-Hungarians and in seek of revenged attacked Serbia.
United States enters the war on April 6 1917 after Germany sunk an American merchant ship and the Zimmerman telegram which was alliance request from Germany to Mexico to go against the United States.