Complex sentence: In the Battle of Dunkirk, he commanded a half-battery that defended the Allies' northern perimeter for four days, before his ammunition ran out; on 1 June 1940, he had to blow up his own guns.
Split into simpler sentences: In the Battle of Dunkirk, he commanded a half-battery that defended the Allies' northern perimeter for four days, before his ammunition ran out. On 1 June 1940, he had to blow up his own guns.