On july 28, 1914, austria-hungary declared war on serbia. russia mobilized to assist serbia. ultimately, and joined russia in an alliance known as the allied powers, while joined and austria-hungary in an alliance known as the central powers.

Answers to fill in the blanks:

France ... Great Britain ... Germany ... the Ottoman Empire.

[Bulgaria was also part of the Central Powers, but you only had one blank there, so I listed the Ottoman Empire.]

Further context/detail:

In the years prior to World War I, there was much military buildup and alliance formation happening among the nations of Europe.  This all served to increase tensions between the nations of Europe.  There was pressure to form alliances with other nations to strengthen a nation's defenses in case there was a war.  The Triple Entente and the Triple Alliance set up in opposition to each other.  The Triple Entente consisted of Britain, France and Russia.  The Triple Alliance consisted of Germany, Austria-Hungary, and Italy. When the war broke out, Italy shifted its loyalties and sided with the Allies (Britain, France, Russia).  Germany and Austria-Hungary picked up Bulgaria and the Ottoman Empire as additional allies during the war, known as the Central Powers.