What is "joins forces"

Usage examples of "joins forces".

Feeling he's learned all he's going to learn at the Academy, Corran Horn leaves and joins forces with Booster Terrik to begin the search for Mirax.

Danilo joins forces with las old enemy, the rogue elf Elaith Craulnober, an elven minstrel with strange abilities, and a dwarf maid with a deadly wit to solve the riddle of the Elfsong.

Often, a vampire sired by another either joins forces with that elder, or completely ignores the connection.