Answer:
Defend, is the right answer.
Step-by-step explanation:
During the First World War, the great powers of Europe divided themselves into two rival alliances- The Triple Entente and the Triple Alliance. The spark came on 28th June when Gavrilo Princip a 19 years old Slav assassinated Archduke Franz Ferdinand, the successor to the throne of Austro-Hungary. For this assassination, Austro-Hungary accused Serbia of providing aid to the assassin and thus forward an ultimatum charging humiliating concessions. As soon as Serbia rejected the proposed ultimatum, Austro-Hungary declared war on Serbia.
On this, the Tsar of Russia came forward to defend Serbia and ordered the Russian Army to mobilize. On the other hand, Austro-Hungary was provided support by Wilhelm II the then Emperor of Germany.