At the age of sixteen, Montecuccoli began as a private soldier under his uncle, Count Ernesto Montecuccoli (died 1633), a distinguished Austrian general. Four years later, after much active service in Germany and the Low Countries, he became a captain of infantry. He was severely wounded at the storming of New Brandenburg, and again in the same year (1631) at the first battle of Breitenfeld, where he fell into the hands of the Swedes.
