Complex sentence: Despite an inauspicious start to the campaign, the Austrian forces, after almost a month, were driven back to Marengo, sparking a great battle on 14 June, which resulted in the Austrian evacuation of Italy.
Split into simpler sentences: Despite an inauspicious start to the campaign, the Austrian forces were driven back to Marengo after nearly a month. There, a a large battle took place on June 14, which resulted in the Austrian evacuation of Italy.