Complex sentence: France declared war on the Habsburg Monarchy of Austria on 20 April 1792, in July 1792 an army under the Duke of Brunswick and composed mostly of Prussians joined the Austrian side and invaded France.
Split into simpler sentences: France declared war on the Habsburg Monarchy of Austria on 20 April 1792. In July 1792, an army under the Duke of Brunswick and composed mostly of Prussians joined the Austrian side and invaded France.