What were the causes of WWI (specifically to war in Europe)?

The underlying causes of WWI led to war in Europe by
causing the countries involved to belive that it was important to go to
war.

The major causes of the war are often listed as
imperialism, militarism, and nationalism.  These forces led countries to feel that it
was important for them to grow in power and to be able to demonstrate this power.  They
wanted, for example, to prove that they were superior to the others because of the
quality of their military.  They wanted to demonstrate their national virtues by having
empires that were as good as those of their rivals.

As the
countries all tried to demonstrate their power, they scared one another.  As Germany,
for example, tried to become stronger, countries like France became alarmed because they
feared that increased German power would be used on them.  The French therefore came to
think that they should fight Germany before Germany became too
powerful.

In this sort of way, the underlying causes of the
war led the countries of Europe to feel that war was in their best
interests.