The princes of Moscow pursued a different system; they no doubt, detested the yoke of the khans as much as their rivals did; but they were aware that, before they could cope with the Tatars, the Russians must be united, and that is was impossible to subject and unite the latter without the assistance of the former. They therefore espoused the daughters of the khans, manifested the utmost submission to the horde, and appeared to be wholly devoted to its interests. Now this policy, which, at the commencement of the Mongol invasion acquired for Alexander Nevski the empire of all Russia, gave it, seventy-four years later, still more completely to Ivan I: for the sway of the Tatars was then more recognised; the Russians were more docile to their yoke; and the cities, which composed the grand principality were more powerful in themselves, and also by comparison with the rest of Russia, which became daily more and more exhausted. The wealth of Ivan I was another cause of the extension of his power. [Sidenote: [1323 A.D.]] The complaints of the prince of Tver, in 1323, prove that Iuri I, grand prince of Moscow, when he undertook to execute the vengeance of his brother-in-law Usbek, against Tver, was also entrusted with the collecting of the tributes; which, however, he retained, instead of sending them to the horde. Ivan Kalita, his brother and successor, profited by this example. Thus it was, that by making themselves lieutenants of the khan, the Muscovite grand princes first became the collectors, and finally the possessors, of the taxes throughout the whole of Russia; and thus they succeeded to all the rights of conquest enjoyed by the Tatars, and to their despotism. There can be no doubt that one of the most copious sources of power to those sovereigns was the periodical census and the perpetual imposts, so alien to feudalism, and especially to a feudalism of princes: these imposts and censuses nothing but the Tatar conquest could have established, and they were inherited by the grand princes. Already, in the first half of the fourteenth century, these taxes had rendered Ivan Kalita rich enough to purchase entire domains and appanages,[14] the protection of Usbek Khan, and the preference of the primate, who removed his residence from Vladimir to Moscow, by which means the latter city became the capital of the empire.