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.
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