2. The Eastern Front: The Eastern Front was the second-largest theatre of operations during World War I, and it was where the Russian Empire and the Central Powers (Germany, Austria-Hungary, and the Ottoman Empire) fought each other. The fighting on the Eastern Front was often very mobile, and it was not as bloody and costly as the fighting on the Western Front.
3. The Italian Front: The Italian Front was the third-largest theatre of operations during World War I, and it was where Italy and the Central Powers fought each other. The fighting on the Italian Front was often very mountainous, and it was very difficult for both sides to make gains.