Complex sentence: Apparently unknown to him, had he advanced north from Büllingen towards Elsenborn, he could have flanked and trapped the 2nd and the 99th Infantry Divisions with possibly a vastly different outcome.
Split into simpler sentences: Apparently unknown to him, had he advanced north from Büllingen towards Elsenborn. He could have flanked and trapped the 2nd and the 99th Infantry Divisions with possibly a vastly different outcome.