About this time another element was introduced into the tangled web of Eastern affairs, which had great influence over the course of events, and over the future conduct of the Anglo-French Allies. Austria now intervened; and her action at this juncture led to important results. This action must be taken briefly into consideration before we can understand the causes that led to the invasion of the Crimea. The geographical position of this Empire necessarily exercises a deciding control over the communications of an army entering the Ottoman territory from the north. Austria, in short, commands the lid of the Turkish box; she can open it, and she can shut it, and prevent the hand of Russia from trying to snatch the prize—Constantinople—which lies at the bottom. In 1853, she held the lid wide open; but in 1854 another course was adopted, more pleasing to the Allies, though not less gratifying to the Tsar. It would not have suited the Government of Vienna to run directly counter to the two maritime Powers of Europe, and to declare themselves openly on the side of Russia; a diversion in Italy might have been serious to their prosperity. So the plain policy of opening the lid could no longer be maintained with safety. They therefore concentrated a force of observation on the south-eastern frontier early in spring, and, having prudently made an offensive and defensive treaty with Prussia, whom they did not trust, they calmly awaited events; and nothing was done till the summer. Austria then prepared for future contingencies, by inducing the Porte to sign a convention (June 14, 1854), by which she undertook to make Russia evacuate the Principalities, and to occupy them herself while hostilities lasted. Still she avoided any dispute with the Emperor Nicholas; she remained inactive, until the fortunes of the war should decide which of the belligerents was going to win in the field. On the 20th of August, however, when victory had declared itself entirely on the side of the Ottoman armies, then, and then only, her forces entered the Principalities, under the agreement that had been already signed, and thereby she rendered several important services to the Russians. She protected their retreat, saved them from disaster, and enabled them to proceed undisturbed into the Crimea, by preventing the Turks from pressing upon them, during their unfortunate march to the Pruth. She became a barrier in the way of the Allies should they deem it necessary to invade Bessarabia, and take a “material guarantee” for the repression of future encroachments in Turkey. And, lastly, she upheld the false prestige of the Tsar’s omnipotence, by making it appear that the Russians had evacuated the Danubian Principalities, not because they were forced to do so by the unaided valour of the Turks, but because the strategic position of the Austrian Empire had obliged them to retire.