Complex sentence: At Reichenberg he encountered Königsegg's Austrian corps; the full Austrian corps consisted of 18.000 infantry and 4.900 cavalry, but only about 10.000 of them had been concentrated at Reichenberg.
Split into simpler sentences: At Reichenberg he encountered Königsegg's Austrian corps. The full Austrian corps consisted of 18,000 infantry and 4,900 cavalry, but only about 10,000 of them had been concentrated at Reichenberg.