Complex sentence: The main theme of the museum is the Third Battle of Ypres, also known as the Battle of Passchendaele, was a great battle in 1917 during the First World War which was fought out by the British, Anzac and Canadian troops and the German army.
Split into simpler sentences: The main theme of the museum is the Third Battle of Ypres, also known as the Battle of Passchendaele. This was a major battle in 1917 during the First World War which was fought out by the British, Anzac and Canadian troops and the German army.