Who? Brodda, Ulfing Chieftan.
In what battle? Battle of Unnumbered Tears
With what faction did he side? Morgoth and his followers.
Brodda, in sympathy with his leader, Uldor the son of Ulfang, sided with Morgoth and his minions against the elves and other free people's of Middle Earth.
Only thing I dont know about is the easy to retreat bit.