Complex sentence: His only defeat in any war, was in the Battle of Porongos, which relaxed the peace negotiations he undertook with the Baron de Caxias, was surprised by the troops of Mouringue, had his troops massacred, including the Black Lancers.
Split into simpler sentences: His only defeat in war was in the Battle of Porongos, which relaxed the peace negotiations he undertook with the Baron de Caxias. He had been surprised by the troops of Mouringue and had his troops defeated, including the Black Lancers.