Although considerable conflict took place outside Europe, the European theatre (also known as the First European War) was the main theatre of operations during World War I and was where the war began and ended. During the four years of conflict, battle was joined by armies of unprecedented size equipped with new mechanized technologies, leaving millions dead or wounded.

The European theatre is divided into four main theatres of operations: the Western Front, the Eastern Front, the Italian Front, and the Balkans Front. Not all of Europe involved in the war. Nor did fighting take place throughout all of the major combatants territory. The United Kingdom was nearly untouched by the war. Most of France was unaffected, as was most of Germany and Italy.

Some large countries in Europe remained neutral for the entire war such as Sweden and Spain – the Great War passed them by without much impact. On the other hand, some countries were conquered (Serbia, Belgium, Romania). Other countries like Russia and the Ottoman Empire saw armies marching over much of their lands, with a great deal of resulting devastation.

Although the United States did join the war, due to Great Britain's control over the Atlantic Ocean, the only fighting for the U.S. Army was in Europe on the Western Front. The American army was transported by ship across the ocean so it could fight the Germans in France.