Complex sentence: The French crossing resulted in the Battle of Aspern-Essling, beginning on 21 May; the more numerous Austrian army faced only a fraction of the "Grande Armée", as Napoleon was unable to bring through the bulk of his forces in time.
Split into simpler sentences: The French crossing resulted in the Battle of Aspern-Essling. Beginning on 21 May, the battle opposed the numerous Austrian army to only a fraction of the "Grande Armée", as Napoleon was unable to bring through the bulk of his forces in time.