On O'Reilly devolved the difficult and trying task of obtaining honourable terms for the capital of the Empire, from an enemy flushed by victory and the pride of a hundred hard-fought fields. He accordingly deputed the Prince of Dietrechstien, the Burgomaster, and the chief citizens to Napoleon, who inveighed bitterly against the obstinacy of the gallant Archduke Ferdinand, but lauded the coolness, bravery, and great presence of mind of the governor, whom he emphatically terms "_le respectable General O'Reilly_," and accepted all the terms proposed by him; but in the fourteenth clause stipulated that O'Reilly should be the bearer of the treaty to his master, to the end that he should honestly and faithfully lay before him the true position of the now half-conquered Austrian Empire--and this duty O'Reilly ably performed. He served in the great battle fought near Aspern on the Marchfeld, during the 21st and 22nd of May, between the French under Napoleon, and the Austrians under the Archduke Charles. In the prince's plan of the attack "to be made upon the hostile army, on its march between Essling and Aspern," it was ordered "that the cavalry brigade under the command of Veesy will be attached to the second column, and the _Regiment O'Reilly_ to the third." This regiment consisted of eight squadrons of Light Dragoons, and the column to which it was attached comprised twenty-two battalions. O'Reilly, with his cavalry, followed the column which marched from Seiring, by the road of Sussenbrunn and Breitenbe. Here O'Reilly, with several troops of Light Horse and Chasseurs formed the advanced guard, which met the enemy's cavalry at three o'clock in the afternoon, near Hirschstettin, while the other columns of the Austrian army drew the French back upon their position between Esslingen and Aspern, and while Lieutenant-General Hohenzollern ordered up his batteries, and the battle became general on all sides. In close column of battalions, the line of the third column was advancing with great bravery, when the French cavalry fell upon them, sabre in hand, with such fury, that they were repulsed, and nearly lost their cannon. At this moment the regiments of Zach, Colloredo, Zetwitz, and the second battalion of the legion of the Archduke Charles, led by Lieutenant-General Brady, an Irish officer, "demonstrated with unparalleled fortitude what the fixed determination to conquer or die is capable of effecting against the most impetuous attacks." The splendid cavalry of France turned both flanks of Brady's column, and penetrating between them, repulsed the Light Horse of O'Reilly, who came up at full speed to succour the soldiers of his countryman. Surrounded, the Regiment O'Reilly were summoned to lay down their arms; but a destructive fire of carbines was the answer to this degrading proposition, and the French cavalry gave way.