World War I took place in various countries across Europe, European colonial bases in South America and Africa, the Ottoman Empire in Asia and in Russia. The war was mainly fought on the Eastern and Western Fronts in Europe as well as at sea. It was the German submarine campaigns at sea that eventually turned United States public opinion, leading to America's involvement in World War I.
World War I initially began on the eastern front in 1914, where Russia invaded East Prussia and Poland. In that same year, battles and mobilizations occurred in the following instances:
- Russian troops mobilized across the Austria-Hungary border while Britain joined its allies in declaring war against Austria-Hungary and Germany.
- Russia invaded the German Empire and Austria-Hungary.
- Germany invaded France and Austria-Hungary invaded Serbia.
- The U.K., with some assistance from Canada, began fighting Germany at sea and in African areas, including Togo.
- The Ottoman Empire of Turkey invaded Russia.
- The Battle of Coronel between Germany and Britain occurred near the Falkland Islands.
In 1915, the territories expanded further:
- German U-boat attacks occurred in the North Atlantic.
- Germany entered Lithuania and Poland to fight against Russia.
- Italy invaded Austria.
- The Allies invaded the Ottoman Empire, while the Ottoman Empire fought Russia in Caucasus.
- The Irish Revolt occurred in Ireland.
- Battles broke out across the Suez Canal and Northern Turkey between the Ottoman Empire and Russia.
- Campaigns erupted in the Balkans.
- Germany began unrestricted submarine warfare, leading to the sinking of the Lusitania and U.S. involvement in the war.
- Germany took hold of the Ukraine and Belarus.
In 1918, the war came to an end and several independent states were formed. This included the Ottoman Empire breaking down, and the liberation of Romania.