Feodor had married the daughter of one of the most illustrious of his nobles.  His father-in-law, a man of peculiar address and capacity, with ability both to conceive and execute the greatest undertakings, soon attained supremacy over the mind of the feeble monarch.  The name of this noble, who became renowned in Russian annals, was Boris Gudenow.  He had the rare faculty of winning the favor of all whom he approached.  With rapid strides he attained the posts of prime minister, commander-in-chief and co-regent of the empire.  A Polish embassador at this time visited Moscow, and, witnessing the extreme feebleness of Feodor, sent word to his ambitious master, Stephen Bathori, that nothing would be easier than to invade Russia successfully; that Smolensk could easily be taken, and that thence the Polish army might find an almost unobstructed march to Moscow.  But death soon removed the Polish monarch from the labyrinths of war and diplomacy.

Boris was now virtually the monarch of Russia, reigning, however, in the name of Feodor.  We have before mentioned that Poland was an elective monarchy.  Immediately upon the death of a sovereign, the nobles, with their bands of retainers, often eighty thousand in number, met upon a large plain, where they spent many days in intrigues and finally in the election of a new chieftain.  Boris Gudenow now roused all his energies in the endeavor to unite Poland and Russia under one monarchy by the election of Feodor as sovereign of the latter kingdom.  The Polish nobles, proud and self-confident, and apprised of the incapacity of Feodor, were many of them in favor of the plan, as Boris had adroitly intimated to them that they might regard the measure rather as the annexing Russia to Poland than Poland to Russia.  All that Boris cared for was the fact accomplished.  He was willing that the agents of his schemes should be influenced by any motives which might be most efficacious.

The Polish diet met in a stormy session, and finally, a majority of its members, instead of voting for Feodor, elected Prince Sigismond, a son of John, King of Sweden.  This election greatly alarmed Russia, as it allied Poland and Sweden by the most intimate ties, and might eventually place the crown of both of those powerful kingdoms upon the same brow.  These apprehensions were increased by the fact that the Crimean Tartars soon again began to make hostile demonstrations, and it was feared that they were moving only in accordance with suggestions which had been sent to them from Poland and Sweden, and that thus a triple alliance was about to desolate the empire.  The Tartars commenced their march.  But Boris met them with such energy that they were driven back in utter discomfiture.