Simeon "the Proud" was the son of Grand Prince
Ivan I of
Russia who came to the throne on his father's death in
1341. His nickname and reference to himself as "Prince of All Russia" both give us an idea of how strong his claim to rule Russia was, after centuries of competition between different cities ruled by different branches of the royal family. The rulers of
Moscow had finally become the unquestioned rulers of all the Russians, even if the Muscovites were still subject to the Mongol conquest. Simeon continued his father's practice of enlarging the amount of land directly subject to Moscow (rather than just part of the Russia ruled by the Grand Prince).
During his reign, Simeon had to deal with Lithuanian invasions, which were especially awkward because the Lithuanians were allied to some of the Mongols. Simeon was able to find an old Russian-Mongol treaty to fall back on, so that the Mongol Khan passed over some Lithuanian prisoners to Russia, and with those hostages Simeon could negotiate a peace with Lithuania.
Simeon died in 1353 at the age of 36, a victim of the Black Death. His younger brother Ivan II succeeded him.