The Joining are an militant race of machines.
According to the Martian Manhunter, the Joining destroyed the race that created them, and since then have launched a campaign of assimilation of technology and knowledge, ravaging half the galaxy. The Joining, made their way to Earth, causing immense destruction.
The Joining were supposedly destroyed after Batman initiated a fail safe protocol within the technology they stole from Wayne Industries. The Joining however did not give up and with the help of Dr. Hugo Strange, consumed the powers of the Justice League. After the heroes took back their powers, the devised a way to shut them down using Hugo Strange who assimilated their entire knowledge.