Answer: Allies fought against the Central Powers - Germany, Austria-Hungary, Bulgaria and the Ottoman Empire.
Step-by-step explanation:
The Allies (Entente Powers) were the coalition that fought against the Central Powers of Germany, Austria-Hungary, the Ottoman Empire and Bulgaria from 1914 to 1918.
- During his visit in Sarajevo, Bosnia, Franz Ferdinand (the heir to the throne of Austria-Hungary), was killed by a Serbian. As a result, Austria-Hungary declared war on Serbia. 
- Russia had an alliance with Serbia, and therefore got involved. In addition, Germany, having an alliance with Austria-Hungary, declared war on Russia. 
- Britain entered the war because of Germany's invasion of Belgium. 
- The Ottoman Empire joined the war in August 1914, upon signing the Turco-German Alliance. 
- Bulgaria declared war on Serbia in October 1915.