How can I argue that nationalism was the most significant cause of WWI?I am writing an essay and trying to explain that nationalism was the most...

The best way to do this is to show how nationalism was
connected to imperialism, militarism and alliances.  If you can show that nationalism
was behind the other causes of WWI, you will be able to say that it was the most
important cause of the war.

So, you might say that
nationalism caused the others in the following
ways:

• It caused imperialism because the
  countries were trying to compete to show that they were all the best.  They wanted to
  have big empires as proof that their nation was
  superior.


• It caused militarism for similar reasons.  Each
  country wanted to show that its military was best and, therefore, that it was a superior
  nation.


• It caused some of the alliances because those
  alliances were based on ethnic identity.  Germany and Austria-Hungary allied because the
  ruling elites of Austria-Hungary were ethnically German.  The Russians and the Serbs
  allied because they were both Slavs.  In this way, nationalism caused the
  alliances.

By pointing these things out, you
can argue that nationalism was the most important cause of the
war.