The key causes of WWI according to the document were militarism, alliances, imperialism, and nationalism in Europe. By 1914, the major European powers were linked in a system of alliances that made a conflict more likely to start and spread. When Archduke Franz Ferdinand of Austria was assassinated by a Serbian nationalist, Austria used it as an excuse to issue harsh demands to Serbia with Germany's support. Russia mobilized in support of its ally Serbia, leading Germany to declare war on Russia and invade neutral Belgium to attack France as outlined in its Schlieffen Plan, drawing Britain into the war in defense of Belgian neutrality set by treaty.