Complex sentence: No sooner had Augereau's corps sallied out when the entire field was engulfed in a sudden snow storm and, in white-out conditions, Augereau's entire corps was lost on the battlefield.
Split into simpler sentences: No sooner had Augereau's corps sallied out when the entire field was engulfed in a sudden snow storm. In white-out conditions, Augereau's entire corps disappeared on the battlefield.