After his sister's fall from favour, Willem joined the Russian army and took part in the Battle of Poltava.
His other sister Matryona Balk had in the meantime become the closest friend of Catherine, whom Peter married in 1712.
A few months later, however, Willem Mons was arrested on charges of peculation (embezzlement) and breach of trust and, after a brief and brutal inquiry by Pyotr Tolstoy, he was publicly beheaded on November 27.
There is a legend that Peter forced his wife to contemplate this gruesome exhibit for hours.
Many courtiers regarded Mons as Catherine's lover and his sister Matryona as their matchmaker.