Complex sentence: In May 1940, Germany launched a surprise attack during its wider invasion of France, and although Belgian forces resisted the German invasion for 18 days, on 28 May 1940, the Belgian army and its commander, King Leopold III, officially surrendered.
Split into simpler sentences: In May 1940, Germany launched a surprise attack on Belgium during its wider invasion of France. On 28 May 1940, the Belgian army and its commander, King Leopold III, officially surrendered.