The first common letter of Lev II and Andrew dates to August 9, 1316[citation needed], in which they renewed the union with the Prussian Crusaders (the Teutonic Order, also known as the Teutonic Knights), whom they promised to defend from the Tatars and "any hostile invader"; the latter mention apparently referred to the Grand Duchy of Lithuania, as that state at the time threatened not only the unity but also the very existence of both the Knights and the Galician state.

It is reported that in the second decade of 14th century, the Lithuanians increased their pressure upon Galicia-Volhynia, eventually wresting away control of Dorohychyn and Berestia.

The years of the brothers' "joint" reign were in spent in more-or-less constant conflict with both the Gediminids of Lithuania and with their own nominal overlords, the Mongol-Tatars of the Golden Horde.

After the deaths of Lev II and his brother two years later, the line of the direct descendants of Roman Mstislavich ended, and the Galician state was left without a ruling dynasty.

But soon more conservative elements amongst the boyars gained the upper hand, and the Galician throne was given to the Mazovian Duke Boleslaw Trojdenowicz (who was related to the deceased royal brothers through the marriage of his father Trojden I of Masovia to their sister Maria).