False Dmitry I was really a Russian man named Yuri Otrepiev. In the early 1600s, he made a public claim to be Dmitry, the son of Ivan the Terrible
who had died in 1591
. Dmitry would have become Tsar
on the death of Fyodor I
if he had lived; instead, Fyodor's brother-in-law Boris Gudenov
took the throne. (Boris is often suspected of having ordered Dmitry's murder.)
To escape Gudenov's agents, Yuri/Dmitry became a monk under the name of Gregory, and actually served the church officials who supported Boris. Then he fled to the Cossack lands of what is now Ukraine, where Russia did not have full control, and eventually to Poland. The Polish, traditionally enemies of Russia, welcomed him, especially when he converted to Catholicism and married a Polish woman of a wealthy and influential family, Marina Mniszek.
In late 1604, with Polish troops, False Dmitry invaded Russia. He was first defeated by a Russian army, but nonetheless gained first the support of 40,000 Cossacks, then a lot of peasants who truly believed he was the rightful tsar, and last many noble families dissatisfied with Boris Gudenov's rule, and his treatment of the popular and powerful Romanov family. In 1605, Boris Gudenov died and his son briefly became Fyodor II, but in June False Dmitry took over Moscow and murdered Fyodor II. The real Dmitry's mother, one of Ivan the Terrible's later wives, came out of a convent and confirmed that this was her son.
However, the Russians were not fond of all the Polish nobles who came along with their new tsar, nor the fact that his wife remained Catholic. The Poles were angry that he wouldn't give them Russian-controlled territory along the border, or introduce more Catholicism into Russia.
False Dmitry was caught in the middle. In 1606, troops led by a Russian boyar of the Shuisky family attacked and killed the tsar; they burned his body and fired the ashes in a cannon in the direction of Poland. Vasiliy Shuisky became the next Russian ruler.