Final answer:
The Allied Powers in World War I initially included France, Britain, and Russia, later joined by Italy, Japan, Belgium, and the United States. The Central Powers consisted of Germany, Austria-Hungary, the Ottoman Empire, and initially Italy, who switched sides in 1915.
Step-by-step explanation:
The Allied and Axis Powers in World War I
The Allied Powers in World War I originally consisted of the Triple Entente: France, Britain, and Russia. Over time, this alliance grew to include additional countries such as Italy (1915), Japan, Belgium, and the United States (1917). On the other side were the Central Powers, initially the Triple Alliance of Germany, Austria-Hungary, and Italy, although Italy switched sides early in the war. Other members of the Central Powers included the Ottoman Empire.
The underlying forces that led to World War I included militarism, a complex system of alliances, imperialism, and nationalism. These forces combined to pull various nations into a large-scale conflict that spanned several years. The allegiances of the countries evolved over the course of the war, with nations such as the United States joining later, tipping the balance in favor of the Allied Powers.