But even as he looked upon his opportunity he saw one further thing that goaded him to an immediate hazard. Upon the north-eastern corner of his strained and bent-back line of battle, against the far, perilous, exposed angle of it, he saw new, quite unexpected hordes of men advancing. It was Ziethen debouching with the head of his First Prussian Army Corps at this latest hour--and Napoleon saw those most distant of his troops ready to yield to the new torrent. The sun, now within an hour of setting, had shone out again. Its light came level down the shallow valley, but all that hollow was so filled with the smoke of recent discharges that the last stroke which Napoleon was now preparing was in part hidden from the Allies upon the hill. That final stake, the only venture left, was to be use of his last reserve and the charge of the Guard. No combat in history, perhaps, had seen a situation so desperate maintained without the order for retreat. Wellington's front, which the French were attacking, was still held unbroken; upon the French flank and rear, though the Fourth Prussian Army Corps were for the moment held, they must inevitably return; more remained to come: they were in the act of pressing upon the only line open to the French for retreat, and now here came Ziethen with his new masses upon the top of all. If, at this hour, just after seven, upon that fatal day, retreat had been possible or advisable to Napoleon, every rule of military art demanded it. He was now quite outnumbered; his exhausted troops were strained up to and beyond the breaking point. To carry such strains too far means in all things, not only in war, an irretrievable catastrophe. But retreat was hardly possible as a military action; it was impossible as a political one. Napoleon could hardly retreat at that hour, although he was already defeated, because the fury and the exhaustion of the combat, its increasing confusion, and the increasing dispersion of its units, made any rapid concentration and organisation for the purposes of a sudden retirement hazardous in the extreme. The doomed body, held closer and closer upon its right flank, menaced more and more on its right rear, now suddenly threatened on its exposed salient angle, would fight on. Though Napoleon had withdrawn from the combat an hour before, when Bülow's 30,000 had struck at his right flank and made his destruction certain; though he had then, while yet he could, organised a retirement, abandoned the furious struggle for La Haye Sainte before it was successful, and covered with his best troops an immediate retreat, that retreat would not have availed his cause. The appearance of the Prussians on his right proved glaringly the nature of his doom. Grouchy--a quarter of his forces--was cut off from him altogether. The enemy, whom he believed to be beyond Grouchy, and pursued by Grouchy, had appeared, upon the contrary, between Grouchy and himself. Now Ziethen too was here.