Simplify the following text for a Grade 10 native English-speaking person.
Original: He was commander during the Battle of the Somme the battle with the highest casualties in British military history , the Third Battle of Ypres and the Hundred Days Offensive which led to the German surrender in 1918 .
Simplified: Most notably he was commander during the Battle of the Somme , the Third Battle of Ypres and the Hundred Days Offensive which led to the German surrender in 1918 .