What was the first major battle of World War I that used trench warfare?

The Battle of Ypres or Battle of the Marne

Explanation:

The Battle of Ypres took place in Ypres, Belgium, and started on April 22, 1915. This was the first major battle to take place in World War 1. In this battle, Germany used mustard poison gas to suffocate the Allied Powers. This is one of the main reasons this battle was so significant. CREDIT: arcgis.com

In the wake of the Battle of the Marne—during which Allied troops halted the steady German push through Belgium and France that had proceeded over the first month of World War I—a conflict both sides had expected to be short and decisive turns longer and bloodier, as Allied and German forces begin digging the first trenches. CREDIT: History.com

answer; ///biological psychologists;