With orders to occupy the line of Riga, and if it was captured, to threaten St. Petersburg, Macdonald marched towards the capital of Livonia, which was occupied by a numerous garrison, whose defensive measures were ably seconded by a British naval force. Napoleon conceived that if the main body of the Russians fell back on St. Petersburg, he would, when following them, be able to effect a junction with the 10th Corps under Macdonald, after which they could push on together; but though the latter burned the suburbs of Riga, his operations against the place were long retarded by the bravery of the besieged. Though not regularly fortified, the town has considerable means of defence, being encircled by an earthen rampart, and having a citadel, while a fortress guards the entrance of the Duna or Dwina. The project of Napoleon became a failure, when the route pursued by the retreating Russians proved different from the one he anticipated. Thus he was obliged to advance after them to Moscow, while Macdonald remained for a time before Riga, on which he could make no impression, though he fought under its walls a series of bloody conflicts, in futile assaults and repulsing desperate sorties. Suspicion of the faith of his Prussian regiments was not his least source of anxiety. When St. Cyr was alarmed that his flanks might be turned by the Russians from Finland, he wrote an urgent letter to Macdonald requesting him to oppose the march of those troops who were led by Wittgenstien and Steinheil, and whose line of march lay in front of the position before Riga; adding that if he (Macdonald) objected to detach any part of his forces from the blockade, to come and assume command of St. Cyr's division in person, and meet this army from Finland. "But Macdonald," adds Count Segur, "did not conceive himself justified in making so important a movement without express orders. He distrusted Yorck, the Prussian general, whom he suspected of intending to deliver up to the Russians his park of siege artillery. He replied, that to defend it was his first and most indispensable duty, and he declined to quit his station." Macdonald's suspicions soon proved correct; for on the 13th December, 1812, when in presence of the enemy, he was abandoned by the whole of the Prussians under General Yorck; and was thus compelled to retire, though resisting with indomitable energy the attack of the Russians, who followed him closely, when sword in hand he sought to hew a passage to the rear. By this time all was lost elsewhere. He survived the perils of that frightful campaign, in which out of 300,000 soldiers, who, in June, passed the Niemann in all the pomp of war and pride of former victories, scarcely 50,000 escaped out of Russia; and of these the greater number had suffered so dreadfully from wounds, hunger and frost, as to be quite unfit for future service.