Complex sentence: Once his scouts reported Forster's retreat, Arnold gave chase, reaching Fort Senneville (which he burned) on May 26, just as Forster's men were landing on the far shore at Quinze-Chênes.
Split into simpler sentences: Once his scouts reported Forster's retreat, Arnold gave chase. He reached Fort Senneville (which he burned) on May 26, just as Forster's men were landing on the far shore at Quinze-Chênes.