 Complete the following phrase:  A period of relative peace followed during the more prosperous and prestigious rule of Vasile Lupu. He took the throne as a boyar appointee in 1637 and began battling his rival Gheorghe Ștefan, as well as the Wallachian prince Matei Basarab. However, his invasion of Wallachia, with the backing of Cossack Hetman Bohdan Khmelnytsky, ended in disaster at the 