Complex sentence: The Battle of Irún was the critical battle of the Campaign of Guipúzcoa during the War in the North, in the Spanish Civil War where the Nationalist Army captured the city of Irún cutting the northern provinces off from their source of arms and support in France.
Split into simpler sentences: The Battle of Irún was the critical battle of the Campaign of Guipúzcoa during the War in the North, in the Spanish Civil War where the Nationalist Army. The capture of Irún cut the northern provinces off from their source of arms and support in France.